Fix #421: Document Color Mode can put RGB greys on the black plate

Switching to CMYK separates RGB greys through the CMYK profile into
four-colour greys and a rich black, as Illustrator does; only Gray
colours stay on the black plate. The command catalogue said "greys stay
greys", which read as if R = G = B colours went to K only.

- file.documentColorMode (and its alias) and document.open {colorMode}
  take grays: "profile" (default, unchanged) | "black". With "black",
  RGB greys with R = G = B convert to K only, K = their grey value (the
  inks Edit Colors > Convert to Grayscale before the switch gives);
  other colours convert as before. Swatches, tints, gradient stops and
  pattern tiles follow the same rule.
- Pattern tiles are now converted with the rest of the document (they
  kept RGB colours in a CMYK document before), as the command said.
- The command catalogue, docs/mcp.md and the ROADMAP say what happens to
  Gray colours and RGB greys.
- Tests: the profile path (four-colour greys, rich black, pattern tiles,
  Gray kept), grays: "black" over art, swatches, tints, gradients and
  patterns, document.open, and the issue's file over MCP through a PDF
  export and back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ROADMAP.md

@@ -4,7 +4,7 @@ VectorCraft is a clean-room, open-source, pure-Rust reimplementation of the Adob
 
 This file tracks **how far we are and what's left**. Time estimates are wall-clock hours of continuous Claude Opus 5.5 agent work (including builds and the CI gate), given both for **one agent** and for **4–6 parallel agents** on disjoint crates. They are counted from the remaining work (see [Parity estimate](#parity-estimate)), calibrated against measured throughput, and updated as work lands.
 
-_Last updated: 2026-10-08 (after M4.14–M4.98, M8.1–M8.20, M14.4–M14.7, the MCP prompts, resource templates, completions and logging, the 2026-10-07 issue fixes, the Layers panel rework and the 2026-10-08 community contributions: Pen and shape-tool modifiers, path editing, Layers, Artboards, Swatches/Brushes and dialogs, Japanese composition and PDF text, Hebrew/Arabic type, interface languages (Spanish among them), the font menu, saved selections, MCP and EPS, the missing-font export warning (#319), open paths in the Shape Builder (#374), PDF/`.ai` import keeping strokes live and hidden and nested layers (#372), per-corner Live Corners (#381), SVG type opening at the size it draws at (#396), the Wayland clipboard with copied-file paste (#398), ruler guides that can be selected, moved and deleted (#414), two batches of Preferences options that were stored but unused (#394), skeleton reads with `document.find` for large documents over MCP (#416) and anchors dragged with Direct Selection snapping to points, segments and smart guides, clipped SVG layers opening as clipping layers (#420), PDF/`.ai` layers read on pages of tens of millions of operators (#419); see [Honest assessment](#honest-assessment-2026-10-05))._
+_Last updated: 2026-10-08 (after M4.14–M4.98, M8.1–M8.20, M14.4–M14.7, the MCP prompts, resource templates, completions and logging, the 2026-10-07 issue fixes, the Layers panel rework and the 2026-10-08 community contributions: Pen and shape-tool modifiers, path editing, Layers, Artboards, Swatches/Brushes and dialogs, Japanese composition and PDF text, Hebrew/Arabic type, interface languages (Spanish among them), the font menu, saved selections, MCP and EPS, the missing-font export warning (#319), open paths in the Shape Builder (#374), PDF/`.ai` import keeping strokes live and hidden and nested layers (#372), per-corner Live Corners (#381), SVG type opening at the size it draws at (#396), the Wayland clipboard with copied-file paste (#398), ruler guides that can be selected, moved and deleted (#414), two batches of Preferences options that were stored but unused (#394), skeleton reads with `document.find` for large documents over MCP (#416) and anchors dragged with Direct Selection snapping to points, segments and smart guides, clipped SVG layers opening as clipping layers (#420), PDF/`.ai` layers read on pages of tens of millions of operators (#419) and Document Color Mode putting RGB greys on the black plate on request and converting pattern tiles (#421); see [Honest assessment](#honest-assessment-2026-10-05))._
 
 ## Where we are
 
@@ -135,7 +135,7 @@ grade by behaviour against `plan/illustrator/`, not by whether a menu item exist
 - **Advanced art:** live Blends (steps/distance/smooth colour, editable spine, anchor-targeted Blend tool, faithful interpolation, knockout), Envelope Distort (warp/mesh/top object, Reset, full Envelope Options, mesh handles, Edit Contents; type, images, symbols, gradients, patterns and appearance distorted everywhere), Liquify tools with options and pen pressure, Puppet Warp with rotating pins, Perspective Grid with presets, movable planes and projected type (choosing a perspective tool shows the grid; Hide Grid hides it with the tool still chosen), Gradient Mesh, Shape Builder (stroke-only open paths and lines cut the regions they cross and enclose regions of their own; Alt deletes their pieces; filled open paths count as closed), Live Paint (every path an edge, open lines included), Image Trace (12 presets), pattern swatches with pattern editing mode, live Repeat (radial/grid/mirror).
 - **Paint and appearance (M3):**
   - **Swatches:** names follow the colour model and are unique; Swatch Options (process/spot, Global, Gray/RGB/HSB/CMYK/Web, live preview); editing a global or spot swatch recolours every linked fill, stroke and text run in one undo step; multi-select and delete with confirmation; New Swatch and New Color Group dialogs (a group from the selected artwork's colours); swatches in colour groups convert, separate and count like any other. The Swatches panel selects ranges and whole groups, has a find field, and drags swatches to reorder, regroup or paint art; Add Used Colors, Select All Unused, Merge, Ungroup and Sort by Kind; a CMYK document keeps applied colours in CMYK. Swatch libraries: nine generated colour libraries and five gradient libraries in a library panel, saved and loaded as `.vcswatches`, `.gpl` or CSS. Tints of global and spot colours ("Name 40%") stay linked and separate as a percentage of their plate. Spot colours can be defined in Lab (Spot Colors options). A built-in [Registration] swatch prints on every plate.
-  - **Colour:** a Color Picker (colour field, H/S/B/R/G/B channel slider, HSB/RGB/Lab/CMYK fields, hex, web-only snapping, out-of-gamut correction); the Color panel follows each colour's own model and Alt-click paints the other proxy; proxies look through groups and show "?" for mixed paint; Invert, Complement, Apply Last Color (`,`) and Apply Last Gradient (`.`) keep the colour model; every paint command feeds the recent colours. The Color Guide has 21 harmony rules, Steps and Variation options and Limit to Library. Adjust Color Balance (its Global mode shifts tints) and Saturate preview live; Edit Colors and Recolor also reach meshes, images and patterns. Recolor Artwork has colour reduction, five recolour methods, presets, Limit to Library, a colour-wheel Edit tab and Edit Color Group; a local Color Themes panel saves five-colour themes. Fill/Stroke chips open swatch and mixer popovers, panels have their shortcuts, and Eyedropper Options pick up and apply chosen attributes (Alt and Shift+Alt, image pixels). Fills and strokes can overprint (Attributes panel, Overprint Black options), and CMYK documents blend in inks on screen, in PDF and when flattening.
+  - **Colour:** a Color Picker (colour field, H/S/B/R/G/B channel slider, HSB/RGB/Lab/CMYK fields, hex, web-only snapping, out-of-gamut correction); the Color panel follows each colour's own model and Alt-click paints the other proxy; proxies look through groups and show "?" for mixed paint; Invert, Complement, Apply Last Color (`,`) and Apply Last Gradient (`.`) keep the colour model; every paint command feeds the recent colours. The Color Guide has 21 harmony rules, Steps and Variation options and Limit to Library. Adjust Color Balance (its Global mode shifts tints) and Saturate preview live; Edit Colors and Recolor also reach meshes, images and patterns. Recolor Artwork has colour reduction, five recolour methods, presets, Limit to Library, a colour-wheel Edit tab and Edit Color Group; a local Color Themes panel saves five-colour themes. Fill/Stroke chips open swatch and mixer popovers, panels have their shortcuts, and Eyedropper Options pick up and apply chosen attributes (Alt and Shift+Alt, image pixels). Fills and strokes can overprint (Attributes panel, Overprint Black options), and CMYK documents blend in inks on screen, in PDF and when flattening. Document Color Mode converts the art, symbols, pattern tiles and swatches through the colour settings: Gray stays Gray, RGB greys separate into four-colour greys and a rich black as Illustrator does, or go on the black plate only with `grays: "black"` (also for `document.open {colorMode}`).
   - **Gradients:** lossless, validated gradient params; gradients follow rotate, reflect, shear, non-uniform scale and distortions; the Gradient tool edits the fill or stroke of the active proxy, type included; an interactive on-canvas annotator (move, length/angle, rotate, add/move/delete/duplicate stops, midpoints, a stop popover, Delete and arrow keys). The Gradient panel has the Fill/Stroke proxy, a gradient-swatch dropdown with Save to Swatches, midpoint locations, Alt-drag to copy or swap stops, swatch drops on the ramp and a stop eyedropper; copied appearances and graphic styles place gradients on each target's own bounds; gradient handles snap and honour Constrain Angle. Gradient stops stay linked to global and spot swatches. Freeform gradients (points and lines) are edited on the canvas; radial gradients have an extent ellipse, an aspect handle and an off-centre focal point (SVG `fx`/`fy`, PDF two-point radials). A stroke's gradient runs within, along or across it. Object ▸ Expand turns gradients into N objects or a gradient mesh.
   - **Strokes:** one stroke-geometry module shared by the canvas, SVG, PDF and Outline Stroke; dotted lines (zero-length dashes draw dots or squares); arrowheads with hollow outlines, tip-on-end or extend-past-end alignment and one opacity for line and head. SVG and PDF export arrowheads, width profiles, brushes and aligned strokes exactly as the canvas draws them; Outline Stroke and the live Outline Stroke effect outline every stroke item as drawn (dashes, heads, profiles, alignment); dashes can be fitted to corners and path ends; width profiles carry through dashes, and their corners take the join. The Stroke panel edits type's character strokes, follows the stroke units, shows mixed values blank and opens from the Control bar. A width profile library, thirty more arrowheads, and Width Point Edit with multi-select and discontinuous points. Visual bounds and clicks cover the whole painted stroke. Scale Strokes & Effects, Scale Corners and Use Preview Bounds work everywhere, and new art inherits stroke options, a graphic style or the whole last appearance.
   - **Appearance:** the Appearance panel's selected row drives every paint, stroke, gradient and transparency edit (`item` on the commands); effects apply to one fill or stroke; per-item Opacity popups; Mixed Appearances, Layers target dots and basic-appearance rules. Effects can be reordered, moved between items and copied by dragging; Show All Hidden Attributes; clicking an effect edits it in place; the object thumbnail drags onto art; effects render on type, images, symbols, blends, envelopes, meshes and repeats, and per-item raster effects export. Groups, layers and type have their own fills, strokes and effects (Contents and Characters rows); the Layers panel's target circles target layers, groups and objects; Expand Appearance turns fills, strokes, effects and brushes into objects.

crates/engine/src/cmd/colormgmt.rs

@@ -8,9 +8,11 @@
 //! become active when assigned.
 
 use std::collections::BTreeMap;
+use std::sync::Arc;
 
 use serde_json::{Value, json};
 use vectorcraft_color::cms::{self, ColorSettings, Intent, Model, ProofTarget};
+use vectorcraft_color::recolor::Neutral;
 use vectorcraft_color::{Color, Paint};
 use vectorcraft_doc::ColorMode;
 use vectorcraft_render::proof;
@@ -55,7 +57,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Convert Document Color Mode",
             [],
             None,
-            "same as file.documentColorMode: {mode: \"cmyk\"|\"rgb\", convert?: true, intent?} → {changed} (an alias kept for older scripts; the command palette leaves it out)",
+            "same as file.documentColorMode: {mode: \"cmyk\"|\"rgb\", convert?: true, intent?, grays?} → {changed} (an alias kept for older scripts; the command palette leaves it out)",
             has_doc,
             convert_mode
         ),
@@ -232,6 +234,28 @@ fn assign_profile(s: &mut Session, p: &Value) -> Result<Value> {
     Ok(json!({"rgb": rgb, "cmyk": cmyk}))
 }
 
+/// How Document Color Mode converts neutral RGB greys (R = G = B) to CMYK (the `grays` param).
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub enum Grays {
+    /// Through the CMYK profile like any colour: four-colour greys and a rich black.
+    #[default]
+    Profile,
+    /// On the black plate only: K is the grey's ink percentage (as Convert to Grayscale and then
+    /// to CMYK would give).
+    Black,
+}
+
+impl Grays {
+    /// The `grays` param of `cmd`: `"profile"` (default) or `"black"`.
+    pub(crate) fn param(cmd: &str, p: &Value) -> Result<Self> {
+        match str_param(p, "grays").map(str::to_ascii_lowercase).as_deref() {
+            None | Some("profile") => Ok(Grays::Profile),
+            Some("black") => Ok(Grays::Black),
+            Some(g) => Err(bad(cmd, format!("grays must be \"profile\" or \"black\", not `{g}`"))),
+        }
+    }
+}
+
 /// File → Document Color Mode (`file.documentColorMode` and its alias
 /// `object.convertDocumentColorMode`): set the mode and, unless `convert: false`, convert every
 /// colour through the colour settings.
@@ -243,6 +267,7 @@ pub(crate) fn convert_mode(s: &mut Session, p: &Value) -> Result<Value> {
         _ => return Err(bad(C, "mode must be \"cmyk\" or \"rgb\"")),
     };
     let intent = intent_param(p, C)?;
+    let grays = Grays::param(C, p)?;
     // Choosing the mode the document already has (the checked menu item) is not an edit.
     if s.doc()?.doc.color_mode == mode {
         return Ok(json!({ "changed": 0 }));
@@ -250,18 +275,20 @@ pub(crate) fn convert_mode(s: &mut Session, p: &Value) -> Result<Value> {
     let convert = bool_or(p, "convert", true);
     let mut changed = 0usize;
     s.edit("Document Color Mode", |d, _| {
-        changed = set_color_mode(d, mode, convert, intent);
+        changed = set_color_mode(d, mode, convert, intent, grays);
         Ok(())
     })?;
     Ok(json!({ "changed": changed }))
 }
 
-/// Put `d` in colour `mode` and, with `convert`, convert every colour of its art, symbols and
-/// swatches through the colour settings (`intent`, default: theirs) → how many colours changed.
-/// Greys stay greys (they print on the black plate in either mode); Lab colours (spot colour
+/// Put `d` in colour `mode` and, with `convert`, convert every colour of its art, symbols,
+/// pattern tiles and swatches through the colour settings (`intent`, default: theirs) → how many
+/// colours of the art, symbols and patterns changed. Gray colours stay Gray (they print on the
+/// black plate in either mode); RGB greys are colours like any other and separate through the
+/// CMYK profile, unless `grays` puts them on the black plate. Lab colours (spot colour
 /// definitions) are device independent and fit either mode. Document Color Mode and
 /// `document.open {colorMode}` both convert this way.
-pub(crate) fn set_color_mode(d: &mut vectorcraft_doc::Document, mode: ColorMode, convert: bool, intent: Option<Intent>) -> usize {
+pub(crate) fn set_color_mode(d: &mut vectorcraft_doc::Document, mode: ColorMode, convert: bool, intent: Option<Intent>, grays: Grays) -> usize {
     d.color_mode = mode;
     if !convert {
         return 0;
@@ -269,15 +296,25 @@ pub(crate) fn set_color_mode(d: &mut vectorcraft_doc::Document, mode: ColorMode,
     let c = cms::active();
     let intent = intent.unwrap_or(c.settings().intent);
     let model = if mode == ColorMode::Cmyk { Model::Cmyk } else { Model::Rgb };
-    let conv = |col: &Color| if matches!(col, Color::Gray { .. } | Color::Lab { .. }) { *col } else { c.convert(col, model, intent) };
+    let k_only = model == Model::Cmyk && grays == Grays::Black;
+    let conv = |col: &Color| match col {
+        Color::Gray { .. } | Color::Lab { .. } => *col,
+        // Grey ink percentage, then K only (a Gray colour's CMYK).
+        Color::Rgb { .. } if k_only && Neutral::of(col).is_some() => c.convert(&c.convert(col, Model::Gray, intent), Model::Cmyk, intent),
+        _ => c.convert(col, model, intent),
+    };
     let mut changed = 0usize;
-    proof::map_document_colors(d, &mut |col, _| {
+    let mut count = |col: &Color, _: proof::Link| {
         let n = conv(col);
         if n != *col {
             changed += 1;
         }
         n
-    });
+    };
+    proof::map_document_colors(d, &mut count);
+    for a in d.patterns.iter_mut().flat_map(|p| &mut p.art) {
+        proof::map_node_colors(Arc::make_mut(a), &mut count);
+    }
     for sw in d.swatches_iter_mut() {
         match &mut sw.paint {
             Paint::Solid { color, .. } => *color = conv(color),

crates/engine/src/cmd/docmenu.rs

@@ -189,7 +189,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Document Color Mode",
             ["File", "Document Color Mode"],
             None,
-            "{mode: \"cmyk\"|\"rgb\", convert?: true (convert every colour of the art, symbols and swatches through the colour settings; swatch links kept; greys stay greys), intent?} → {changed}",
+            "{mode: \"cmyk\"|\"rgb\", convert?: true (convert every colour of the art, symbols, pattern tiles and swatches through the colour settings; swatch links kept; Gray colours stay Gray, on the black plate), intent?, grays?: \"profile\" (default: RGB greys separate through the CMYK profile like any colour, into four-colour greys and a rich black) | \"black\" (to CMYK, RGB greys with R = G = B go on the black plate only, K = their grey value)} → {changed}",
             has_doc,
             super::colormgmt::convert_mode
         ),

crates/engine/src/cmd/fileio/load.rs

@@ -178,7 +178,7 @@ pub fn load_with(name: &str, bytes: &[u8], opts: &LoadOptions) -> Result<Loaded>
         _ => return Err(err(format!("{} files can't be opened yet", format.label))),
     };
     if let Some(mode) = opts.color_mode.filter(|m| *m != doc.color_mode) {
-        super::super::colormgmt::set_color_mode(&mut doc, mode, true, None);
+        super::super::colormgmt::set_color_mode(&mut doc, mode, true, None, opts.grays);
     }
     // Imports are named after the file; a native document keeps its own title (the tab shows the
     // file name once it has a path).

crates/engine/src/cmd/fileio/mod.rs

@@ -78,7 +78,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Open Document",
             [],
             None,
-            "{path} or {name, dataBase64}, PDF/.ai: pages?: \"2-3, 5\" (1-based, default all; one artboard each) | page?: n, cropTo?: bounding|art|crop (default)|trim|bleed|media (the box each artboard gets; bounding: the art's bounds), password? (encrypted PDFs; see document.pdfInfo), textAs?: text (default: point type per line, in the file's fonts by name; missing ones are listed in warnings) | outlines (glyph paths), layers?: true (default: optional content groups become layers with their visibility, print state and lock — art that is off comes in as a hidden layer; other art goes to a layer per page) | false (one layer per page, without the art that is off; Save then asks for a name), colorMode?: rgb|cmyk (the mode the document opens in, its colours converted as file.documentColorMode does; default: the file's — a PDF keeps CMYK, Gray and spot inks (spot swatches at a tint) and opens in CMYK when painted mostly in CMYK), DXF: dxf?: {layout?: \"Model\" (default) | a paper layout's name (document.dxfInfo lists them), unit?, scale? (the ratio: 1 unit of the art = scale drawing units; default: the drawing at 1:1 in its own unit), fit?: false (scale to fit an artboard of fitTo?: [612, 792], turned to the art's orientation), scaleLineweights?: false (lineweights scale with the art), center?: true (false: the drawing's origin on the artboard's bottom-left corner; fitted art's bottom-left corner), mergeLayers?: false (all art on one layer)} → {index, title, format, warnings, restored, missingLinks, modifiedLinks, updatedLinks: [{name, path, ids}]}; any readable format (see document.formats): .vectorcraft/.drawcraft, .svg/.svgz, .pdf/.ai, .ait, ASCII .dxf (each DXF layer with art a layer; blocks symbols; what can't come in is listed in warnings), PNG/JPEG/GIF/WebP/TIFF/BMP (an image opens as a document of its pixel size), .emf/.wmf (one artboard, the picture's frame; records VectorCraft doesn't read are skipped with one warning), .eps and PostScript .ai (one artboard, the bounding box: an EPS file VectorCraft wrote restores the document it carries, restored: true; other files are read by a PostScript interpreter — paths, colours and spot inks, clips, gradients, images, type in the fonts named — and come in as their TIFF preview, with a warning, when their PostScript can't be read). A PDF/.ai/.ait or SVG saved with Preserve Editing restores the native document it carries (restored: true; a PDF only when no pages are picked), unless the file was changed elsewhere since or the data can't be read: then its artwork is imported and the first warning says why. Templates (native templates, .ait) open as a new untitled document; a restored .ai keeps its path (Save writes .ai again). Linked images are read from their files (looked for at their path, then relative to the document): missing ones show their saved preview (links.relink), modified ones are read again only with Preferences › Update Links: Automatically (else links.update). An SVG's <image> files (relative links from the SVG's folder) stay linked, SVG files become art, missing ones a placeholder in their box (a warning and missingLinks)",
+            "{path} or {name, dataBase64}, PDF/.ai: pages?: \"2-3, 5\" (1-based, default all; one artboard each) | page?: n, cropTo?: bounding|art|crop (default)|trim|bleed|media (the box each artboard gets; bounding: the art's bounds), password? (encrypted PDFs; see document.pdfInfo), textAs?: text (default: point type per line, in the file's fonts by name; missing ones are listed in warnings) | outlines (glyph paths), layers?: true (default: optional content groups become layers with their visibility, print state and lock — art that is off comes in as a hidden layer; other art goes to a layer per page) | false (one layer per page, without the art that is off; Save then asks for a name), colorMode?: rgb|cmyk (the mode the document opens in, its colours converted as file.documentColorMode does, grays? as there; default: the file's — a PDF keeps CMYK, Gray and spot inks (spot swatches at a tint) and opens in CMYK when painted mostly in CMYK), DXF: dxf?: {layout?: \"Model\" (default) | a paper layout's name (document.dxfInfo lists them), unit?, scale? (the ratio: 1 unit of the art = scale drawing units; default: the drawing at 1:1 in its own unit), fit?: false (scale to fit an artboard of fitTo?: [612, 792], turned to the art's orientation), scaleLineweights?: false (lineweights scale with the art), center?: true (false: the drawing's origin on the artboard's bottom-left corner; fitted art's bottom-left corner), mergeLayers?: false (all art on one layer)} → {index, title, format, warnings, restored, missingLinks, modifiedLinks, updatedLinks: [{name, path, ids}]}; any readable format (see document.formats): .vectorcraft/.drawcraft, .svg/.svgz, .pdf/.ai, .ait, ASCII .dxf (each DXF layer with art a layer; blocks symbols; what can't come in is listed in warnings), PNG/JPEG/GIF/WebP/TIFF/BMP (an image opens as a document of its pixel size), .emf/.wmf (one artboard, the picture's frame; records VectorCraft doesn't read are skipped with one warning), .eps and PostScript .ai (one artboard, the bounding box: an EPS file VectorCraft wrote restores the document it carries, restored: true; other files are read by a PostScript interpreter — paths, colours and spot inks, clips, gradients, images, type in the fonts named — and come in as their TIFF preview, with a warning, when their PostScript can't be read). A PDF/.ai/.ait or SVG saved with Preserve Editing restores the native document it carries (restored: true; a PDF only when no pages are picked), unless the file was changed elsewhere since or the data can't be read: then its artwork is imported and the first warning says why. Templates (native templates, .ait) open as a new untitled document; a restored .ai keeps its path (Save writes .ai again). Linked images are read from their files (looked for at their path, then relative to the document): missing ones show their saved preview (links.relink), modified ones are read again only with Preferences › Update Links: Automatically (else links.update). An SVG's <image> files (relative links from the SVG's folder) stay linked, SVG files become art, missing ones a placeholder in their box (a warning and missingLinks)",
             always,
             load::open
         ),

crates/engine/src/cmd/fileio/pdfimport.rs

@@ -6,6 +6,7 @@ use serde_json::{Value, json};
 use vectorcraft_doc::{ColorMode, Document};
 use vectorcraft_pdf::{Choice, CropTo, ImportOptions, PdfError, TextAs};
 
+use super::super::colormgmt::Grays;
 use super::super::*;
 use super::{err, source};
 
@@ -22,6 +23,8 @@ pub struct LoadOptions {
     /// The colour mode the document opens in, its colours converted as Document Color Mode does
     /// (default: the file's; a PDF painted mostly in CMYK opens in CMYK).
     pub color_mode: Option<ColorMode>,
+    /// How that conversion separates RGB greys (`grays`).
+    pub grays: Grays,
     /// What a PDF's text becomes.
     pub text_as: TextAs,
     /// A PDF's optional content groups become layers (else one layer per page, without the art
@@ -38,6 +41,7 @@ impl Default for LoadOptions {
             crop: CropTo::default(),
             password: None,
             color_mode: None,
+            grays: Grays::default(),
             text_as: TextAs::default(),
             layers: true,
             dxf: Default::default(),
@@ -47,7 +51,7 @@ impl Default for LoadOptions {
 
 impl LoadOptions {
     /// From command params: `pages` (`"2-3, 5"`, a page number or `"all"`), `page` (one page, when
-    /// `pages` isn't given), `cropTo` (or `crop`), `password`, `colorMode` (`rgb` | `cmyk`),
+    /// `pages` isn't given), `cropTo` (or `crop`), `password`, `colorMode` (`rgb` | `cmyk`) with `grays`,
     /// `textAs` (`text` | `outlines`), `layers` (true | false) and `dxf` (the DXF import options).
     pub fn from_params(cmd: &str, p: &Value) -> Result<Self> {
         let pages = match p.get("pages").filter(|v| !v.is_null()).or_else(|| p.get("page").filter(|v| !v.is_null())) {
@@ -65,13 +69,14 @@ impl LoadOptions {
             Some("cmyk") => Some(ColorMode::Cmyk),
             Some(m) => return Err(bad(cmd, format!("colorMode must be rgb or cmyk, not `{m}`"))),
         };
+        let grays = Grays::param(cmd, p)?;
         let text_as = choice(cmd, "textAs", str_param(p, "textAs"))?;
         let layers = match p.get("layers").filter(|v| !v.is_null()) {
             None => true,
             Some(v) => v.as_bool().ok_or_else(|| bad(cmd, "layers must be true or false"))?,
         };
         let dxf = super::dxfimport::options(cmd, p.get("dxf"))?;
-        Ok(Self { pages, crop, password, color_mode, text_as, layers, dxf })
+        Ok(Self { pages, crop, password, color_mode, grays, text_as, layers, dxf })
     }
 
     /// Does the document read only part of the file, or read it differently (a page range, another

crates/engine/src/tests_menucmds.rs

@@ -1,7 +1,7 @@
 //! Menu long-tail commands (Object/Edit/Select/Type/View/File), driven through `Session::execute`.
 
 use serde_json::{Value, json};
-use vectorcraft_color::Color;
+use vectorcraft_color::{Color, Paint};
 use vectorcraft_doc::{ColorMode, LiveShape, Node, NodeKind, TextKind};
 use vectorcraft_geom::Rect;
 
@@ -859,6 +859,122 @@ fn document_color_mode_converts_colours() {
     assert!(s.execute("file.documentColorMode", &json!({"mode": "lab"})).is_err());
 }
 
+/// Four RGB greys (#000000, #333333, #808080, #e6e6e6, as in issue #421) and a red, a 40% tint of a
+/// grey global swatch, a gradient to grey and a pattern of a grey tile → (session, the five
+/// rectangles, the tinted one, the gradient one).
+fn greys_document() -> (Session, Vec<NodeId>, NodeId, NodeId) {
+    let mut s = session();
+    let ids: Vec<NodeId> = ["#000000", "#333333", "#808080", "#e6e6e6", "#ff0000"]
+        .iter()
+        .map(|hex| {
+            let id = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+            fill(&mut s, id, hex);
+            id
+        })
+        .collect();
+    s.execute("swatch.new", &json!({"name": "Ink", "color": "#808080", "global": true})).unwrap();
+    let tint = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+    s.execute("paint.setFill", &json!({"ids": [tint.0], "swatch": "Ink", "tint": 40})).unwrap();
+    let grad = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+    let stops = json!([{"offset": 0, "color": "#ffffff"}, {"offset": 1, "color": "#333333"}]);
+    s.execute("paint.setFill", &json!({"ids": [grad.0], "gradient": {"kind": "linear", "stops": stops}})).unwrap();
+    let tile = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+    fill(&mut s, tile, "#808080");
+    s.execute("object.pattern.make", &json!({"ids": [tile.0], "name": "Grey Tile", "edit": false})).unwrap();
+    (s, ids, tint, grad)
+}
+
+/// The colour of pattern `name`'s first tile object.
+fn pattern_fill(s: &Session, name: &str) -> Color {
+    s.doc().unwrap().doc.pattern(name).unwrap().art[0].appearance.fill_paint().color().unwrap()
+}
+
+/// K of a colour that prints on the black plate only.
+fn k_only(c: Color) -> f32 {
+    match c {
+        Color::Cmyk { c: 0.0, m: 0.0, y: 0.0, k } => k,
+        other => panic!("not K only: {other:?}"),
+    }
+}
+
+#[test]
+fn document_color_mode_separates_rgb_greys_through_the_profile() {
+    let (mut s, ids, _, _) = greys_document();
+    s.execute("file.documentColorMode", &json!({"mode": "cmyk"})).unwrap();
+    // As Illustrator converts them: four-colour greys and a rich black.
+    let Color::Cmyk { c, m, y, k } = fill_of(&s, ids[2]) else { panic!("not CMYK") };
+    assert!(c > 0.05 && m > 0.05 && y > 0.05 && k > 0.0, "a four-colour mid grey: {c} {m} {y} {k}");
+    let Color::Cmyk { c, k, .. } = fill_of(&s, ids[0]) else { panic!("not CMYK") };
+    assert!(k > 0.9 && c > 0.3, "rich black: {c} {k}");
+    // Pattern tiles are swatches too: their art converts with the rest.
+    assert!(matches!(pattern_fill(&s, "Grey Tile"), Color::Cmyk { .. }), "{:?}", pattern_fill(&s, "Grey Tile"));
+    // Gray colours stay Gray.
+    let mut g = session();
+    let a = rect(&mut g, 0.0, 0.0, 10.0, 10.0);
+    g.execute("paint.setFill", &json!({"ids": [a.0], "color": {"gray": 0.3}})).unwrap();
+    g.execute("file.documentColorMode", &json!({"mode": "cmyk"})).unwrap();
+    assert_eq!(fill_of(&g, a), Color::gray(0.3));
+}
+
+#[test]
+fn document_color_mode_can_put_rgb_greys_on_the_black_plate() {
+    let (mut profile, pids, ..) = greys_document();
+    profile.execute("file.documentColorMode", &json!({"mode": "cmyk"})).unwrap();
+    let (mut s, ids, tint, grad) = greys_document();
+    let e = s.execute("file.documentColorMode", &json!({"mode": "cmyk", "grays": "rich"})).unwrap_err().to_string();
+    assert!(e.contains("grays"), "{e}");
+    s.execute("file.documentColorMode", &json!({"mode": "cmyk", "grays": "black"})).unwrap();
+    let close = |a: f32, b: f32| (a - b).abs() < 0.005;
+    for (id, want) in ids.iter().zip([1.0, 0.8, 0.498, 0.098]) {
+        let k = k_only(fill_of(&s, *id));
+        assert!(close(k, want), "K {k}, want {want}");
+    }
+    // Other colours convert through the profile as before.
+    assert_eq!(fill_of(&s, ids[4]), fill_of(&profile, pids[4]));
+    // Swatches, their tints, gradient stops and pattern tiles follow the same rule.
+    let d = &s.doc().unwrap().doc;
+    assert!(close(k_only(d.swatch("Ink").unwrap().paint.color().unwrap()), 0.498));
+    let Paint::Solid { color, swatch, tint: t } = node(&s, tint).appearance.fill_paint() else { panic!("not solid") };
+    assert!(close(k_only(color), 0.498 * 0.4) && swatch.as_deref() == Some("Ink") && t == 0.4, "{color:?} {swatch:?} {t}");
+    let Paint::Gradient(g) = node(&s, grad).appearance.fill_paint() else { panic!("not a gradient") };
+    let ks: Vec<f32> = g.gradient.stops.iter().map(|st| k_only(st.color)).collect();
+    assert!(close(ks[0], 0.0) && close(ks[1], 0.8), "{ks:?}");
+    assert!(close(k_only(pattern_fill(&s, "Grey Tile")), 0.498));
+    // To RGB, `grays` changes nothing.
+    s.execute("file.documentColorMode", &json!({"mode": "rgb", "grays": "black"})).unwrap();
+    let rgb = s.doc().unwrap().doc.layers.clone();
+    s.execute("edit.undo", &json!({})).unwrap();
+    s.execute("file.documentColorMode", &json!({"mode": "rgb"})).unwrap();
+    assert_eq!(s.doc().unwrap().doc.layers, rgb);
+}
+
+#[test]
+fn opening_in_cmyk_can_put_rgb_greys_on_the_black_plate() {
+    let mut s = session();
+    for hex in ["#000000", "#333333", "#808080", "#e6e6e6", "#ff0000"] {
+        let id = rect(&mut s, 0.0, 0.0, 10.0, 10.0);
+        fill(&mut s, id, hex);
+    }
+    let svg = s.execute("document.serialize", &json!({"format": "svg"})).unwrap()["text"].as_str().unwrap().to_string();
+    let data = vectorcraft_format::base64_encode(svg.as_bytes());
+    let mut o = Session::new();
+    o.execute("document.open", &json!({"name": "greys.svg", "dataBase64": data, "colorMode": "cmyk", "grays": "black"})).unwrap();
+    let d = &o.doc().unwrap().doc;
+    assert_eq!(d.color_mode, ColorMode::Cmyk);
+    let mut fills = vec![];
+    for l in &d.layers {
+        l.walk(&mut |n| fills.extend(n.children().is_none().then(|| n.appearance.fill_paint().color()).flatten()));
+    }
+    assert_eq!(fills.len(), 5, "{fills:?}");
+    for (c, want) in fills.iter().zip([1.0, 0.8, 0.498, 0.098]) {
+        let k = k_only(*c);
+        assert!((k - want).abs() < 0.005, "K {k}, want {want}");
+    }
+    assert!(matches!(fills[4], Color::Cmyk { m, y, .. } if m > 0.8 && y > 0.8), "red separates as usual: {:?}", fills[4]);
+    let e = Session::new().execute("document.open", &json!({"name": "greys.svg", "dataBase64": data, "grays": "k"})).unwrap_err().to_string();
+    assert!(e.contains("grays"), "{e}");
+}
+
 #[test]
 fn file_info_sets_title() {
     let mut s = session();

crates/mcp/src/tests_fileio.rs

@@ -145,3 +145,55 @@ fn pdf_pages(pdf: &[u8]) -> usize {
         .unwrap();
     h.session.doc().unwrap().doc.artboards.len()
 }
+
+/// Issue #421 over MCP: RGB greys switched to CMYK with `grays: "black"` print on the black plate
+/// only, in the document and in the exported PDF.
+#[test]
+fn document_color_mode_puts_rgb_greys_on_the_black_plate() {
+    let svg = tmp("greys.svg");
+    std::fs::write(
+        &svg,
+        r##"<svg xmlns="http://www.w3.org/2000/svg" width="200pt" height="50pt" viewBox="0 0 200 50">
+<rect x="0" y="0" width="50" height="50" fill="#000000"/><rect x="50" y="0" width="50" height="50" fill="#333333"/>
+<rect x="100" y="0" width="50" height="50" fill="#808080"/><rect x="150" y="0" width="50" height="50" fill="#e6e6e6"/>
+</svg>"##,
+    )
+    .unwrap();
+    let want = [1.0, 0.8, 0.498, 0.098];
+    let k_only = |h: &Headless| -> Vec<f32> {
+        let mut ks = vec![];
+        for l in &h.session.doc().unwrap().doc.layers {
+            l.walk(&mut |n| {
+                if n.children().is_none()
+                    && let Some(c) = n.appearance.fill_paint().color()
+                {
+                    let v = serde_json::to_value(c).unwrap();
+                    assert!(v["model"] == "cmyk" && v["c"] == 0.0 && v["m"] == 0.0 && v["y"] == 0.0, "not K only: {v}");
+                    ks.push(v["k"].as_f64().unwrap() as f32);
+                }
+            });
+        }
+        ks
+    };
+    let close = |ks: &[f32]| ks.len() == want.len() && ks.iter().zip(want).all(|(k, w)| (k - w).abs() < 0.005);
+    let mut h = Headless::new();
+    text(&call_tool(&mut h, "open_file", &json!({"path": svg})));
+    let r = text(&call_tool(&mut h, "run_command", &json!({"command": "file.documentColorMode", "params": {"mode": "cmyk", "grays": "black"}})));
+    assert_eq!(r["changed"], 4);
+    let ks = k_only(&h);
+    assert!(close(&ks), "{ks:?}");
+    // The PDF carries the same inks (DeviceCMYK opens as CMYK).
+    let pdf = text(&call_tool(&mut h, "export", &json!({"format": "pdf"})));
+    let mut back = Headless::new();
+    back.call("engine.execute", json!({"command": "document.open", "params": {"name": "greys.pdf", "dataBase64": pdf["dataBase64"]}})).unwrap();
+    let ks = k_only(&back);
+    assert!(close(&ks), "{ks:?}");
+    // document.open takes the same option.
+    let mut o = Headless::new();
+    let r = call_tool(&mut o, "run_command", &json!({"command": "document.open", "params": {"path": svg, "colorMode": "cmyk", "grays": "black"}}));
+    text(&r);
+    let ks = k_only(&o);
+    assert!(close(&ks), "{ks:?}");
+    let bad = call_tool(&mut o, "run_command", &json!({"command": "file.documentColorMode", "params": {"mode": "rgb", "grays": "rich"}}));
+    assert!(bad.is_error, "{bad:?}");
+}

docs/mcp.md

@@ -269,8 +269,12 @@ without a path), `file.newFromTemplate {path}`, `file.revert` and `file.formatOp
 with the values a save would use). Without a path the save commands return `{dataBase64, name, folder?, warnings}`. A document saved as SVG or PDF
 remembers its options, so the next save reuses them. A PDF opened with `pages`, `cropTo` or `password` is only part
 of the file, so Save asks for a new name instead of writing it back.
-`file.documentColorMode {mode, convert?, intent?}` switches the document colour mode, converting its colours
-through the colour settings (`object.convertDocumentColorMode` is an alias kept for older scripts).
+`file.documentColorMode {mode, convert?, intent?, grays?}` switches the document colour mode, converting the colours of
+the art, symbols, pattern tiles and swatches through the colour settings (`object.convertDocumentColorMode` is an
+alias kept for older scripts). Gray colours stay Gray and print on the black plate only. RGB greys are colours like any
+other: to CMYK they separate through the profile into four-colour greys and a rich black, as Illustrator converts
+them. `grays: "black"` puts RGB greys with R = G = B on the black plate instead, K = their grey value (the inks Edit Colors ›
+Convert to Grayscale before the switch would give); other colours convert as usual.
 
 SVG import keeps what the canvas edits live. `<pattern>` becomes a pattern swatch. A `<symbol>` with `<use>` becomes a
 symbol (named after its `data-name`, else its id) with an instance per `<use>`; a `<use>` that shows it differently
@@ -370,7 +374,8 @@ and Separation and DeviceN inks become spot swatches the art links to at its tin
 (DeviceCMYK, or ICC-based with four components) keep their ink amounts: a CMYK JPEG as it is, any other as a CMYK TIFF
 (masked CMYK images, and ones with 1, 2 or 4 bits per sample, open in RGB with a warning). Placing a CMYK TIFF keeps it
 CMYK too.
-`colorMode: "rgb" | "cmyk"` opens any file in that mode instead, its colours converted as `file.documentColorMode` does:
+`colorMode: "rgb" | "cmyk"` opens any file in that mode instead, its colours converted as `file.documentColorMode` does
+(`grays` too):
 
 ```json
 {"name":"run_command","arguments":{"command":"document.pdfInfo","params":{"path":"/tmp/brochure.pdf","thumbnail":2,"cropTo":"trim"}}}