diff --git a/AGENTS.md b/AGENTS.md
index 028c6d837e..3fa440d0d6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -5,7 +5,7 @@
 ## Design System
 
 See **[DESIGN.md](./DESIGN.md)** for the docs site's design system: tokens, typography utilities,
-dark-mode rules, and STRICT code rules.
+dark-mode rules, diagram roles, and STRICT code rules.
 
 When you write or modify any styling, read `DESIGN.md` first. The rules there override defaults —
 no hex literals, no Chakra references, no `--color-mergify-blue` references, and components should
@@ -13,6 +13,12 @@ use semantic tokens by default so dark mode works automatically. Product-specifi
 (callouts, brand-colored buttons, product pills) may use `var(--color-*)` primitives directly as a
 narrow documented exception — see `DESIGN.md` for the policy.
 
+**Diagrams never name a color.** A Graphviz fence, `<GitGraph>` and `<StackMapping>` all name a
+*role* — `queued`, `pending`, `merged`, `failed`, `config`, `mergify`, `datastore`, `external`,
+`batch`, `muted`, `chrome` — and the page resolves it at paint time, which is what makes them
+theme-aware. `pnpm check:diagram-tokens` fails CI on a color in a diagram. See the "Diagrams"
+section of `DESIGN.md` before adding or editing one.
+
 ## Quick Facts
 
 - **Framework**: Astro 5.x with MDX
@@ -43,6 +49,7 @@ pnpm test              # Run Vitest unit tests
 
 # Safety
 pnpm check:internal-leaks  # Scan docs for leaked internal information
+pnpm check:diagram-tokens  # Scan diagrams for hardcoded colors
 ```
 
 ## Key Directories
diff --git a/DESIGN.md b/DESIGN.md
index 91bfd296b3..bf7bdb4426 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -17,6 +17,8 @@ source files listed below; the code is the source of truth.
 | Typography utility classes | [`src/styles/typography.css`](./src/styles/typography.css) |
 | Global content rules + section-accent system | [`src/styles/index.css`](./src/styles/index.css) |
 | Product accent colors (canonical source) | [`mergify.com/DESIGN.md`](../mergify.com/DESIGN.md) — "Product Accent Colors" section |
+| Diagram roles + paint rules | [`src/styles/index.css`](./src/styles/index.css) — the `.dg` block |
+| Diagram SVG post-processing | [`src/util/diagramSvg.ts`](./src/util/diagramSvg.ts) |
 | Reusable Astro components | [`src/components/`](./src/components/) |
 | Page layout templates | [`src/layouts/`](./src/layouts/) |
 | Icons (Phosphor bold + custom SVGs) | Inline `?raw` imports; no icon component abstraction yet |
@@ -210,6 +212,90 @@ surrounding surface contrast changes instead.
 4. Document the new token in this file under the appropriate section.
 5. Consume the semantic token — never the primitive — in components.
 
+## Diagrams
+
+Diagrams were the one surface that escaped this document, and it showed: 63 distinct colors across
+18 pages, only 10 of which existed anywhere in `tokens.css`, in four unrelated dialects, with a
+dark mode that was a hardcoded list of six strings a build-time plugin matched against. This
+section exists so it does not happen again, and `pnpm check:diagram-tokens` enforces the one rule
+that matters.
+
+**A diagram never names a color.** It names a *role*, and the page resolves the color at paint
+time. Graphviz copies a `class` attribute verbatim into the SVG, so a role survives the render and
+CSS can do the rest — which is why dark mode needs no second render and no second palette.
+
+### Roles
+
+| Role | What it means |
+| --- | --- |
+| `queued` | in the queue, waiting its turn |
+| `pending` | running now — validating, testing |
+| `merged` | merged, passed, done |
+| `failed` | failed, dequeued, cascaded out |
+| `config` | configuration and inputs |
+| `mergify` | a component we run |
+| `datastore` | Postgres, Redis, storage |
+| `external` | GitHub, CI, third parties — not us |
+| `batch` | a grouping container |
+| `muted` | skipped, dashed, de-emphasized |
+| `chrome` | edges, arrowheads, captions |
+
+`plain` is not a role: it marks an element as a caption rather than a box, and combines with one
+(`class="merged plain"`). The plugin infers it for a shape Graphviz drew with no border, so
+`shape=plaintext` needs nothing.
+
+`queued` and `mergify` deliberately share the Merge Queue teal. Same color, two names, because
+"waiting in the queue" and "a service we run" are the same idea on two different kinds of diagram,
+and a role name that lies is worse than a duplicated accent. Adding a role costs three lines: an
+accent in `theme.css` (both blocks), a `.dg .<role>` rule in `index.css`, and an entry in
+`DIAGRAM_ROLES` in `src/util/diagramSvg.ts`.
+
+### Diagram kinds
+
+A fence opts into a layout with its class — `` ```dot class="queue" `` — which sets direction and
+spacing, never color. A fence that names none lays itself out.
+
+| Kind | For |
+| --- | --- |
+| `queue` | left-to-right pipelines: queues, batches, CI fan-out |
+| `flow` | top-to-bottom state machines and decision trees |
+| `arch` | boxes and wires, orthogonal routing, wide labels |
+
+### How a role becomes a color
+
+Each role carries one accent. `index.css` mixes it against the page background to produce a
+surface, a border and a label, so a diagram tints toward whatever surface it sits on. Dark mode
+changes only how much (`--dg-tint`, `--dg-ink`, `--dg-edge-lift`) and the neutrals — never the
+product accents, per "Product palette does not flip" above. Borders do lift toward white on dark:
+that is a value *derived* from an accent, not a flipped accent, and a `--color-green-700` outline
+on the dark page surface is otherwise too dim to read.
+
+A container is mixed at a fraction of a node's tint (`--dg-tint-cluster`) so a node of the same
+role reads as sitting on top of it rather than dissolving into it.
+
+### The four surfaces
+
+All four emit the same `node` / `edge` / `cluster` groups and the same role classes, and all four
+are painted by the `.dg` block in `index.css`:
+
+- `` ```dot `` fences, rendered by `plugins/remark-graphviz.ts`
+- `<GitGraph>` graph mode, also Graphviz
+- `<GitGraph>` linear mode, hand-placed SVG
+- `<StackMapping>`, hand-placed SVG
+
+`src/util/diagramSvg.ts` holds what the two Graphviz surfaces share. Before this, each had its own
+`COLORS` map and its own dark-mode string-replace, which is how they drifted apart.
+
+### Writing one
+
+- Name a role, never a color. `PR1 [class="queued"];`, not `PR1 [fillcolor="#347D39"];`
+- Keep cluster labels short. Graphviz sizes a container to fit the label it measured in Helvetica,
+  and the page paints it in Inter at weight 600 — roughly 4% wider. Node labels have margin to
+  absorb the drift; cluster labels do not.
+- Do not set `fontsize`, `nodesep` or `ranksep` unless the diagram genuinely needs it. Every fence
+  gets a shared set of defaults, and a fence that overrides them stops matching its neighbours.
+- `none` and `transparent` are shape, not color, and the lint knows the difference.
+
 ## Typography
 
 Two font families:
@@ -334,7 +420,8 @@ These rules apply when writing or generating component, style, or page code.
   `var(--color-*)` primitives are only allowed inside `tokens.css` and `theme.css`.
 - **Inline SVG `fill`/`stroke`**: the only place raw hex is permitted outside `tokens.css`. Use
   `currentColor` wherever possible; fall back to a `var(--color-*)` primitive only when the SVG
-  must carry its own color independent of the theme.
+  must carry its own color independent of the theme. **Diagrams are not covered by this
+  exemption** — they name a role and CSS resolves it; see "Diagrams" above.
 - **Typography**: use the utility classes (`heading-section`, `text-subtitle`, etc.) on custom
   pages. Do not compose ad-hoc `font-size` / `font-weight` / `letter-spacing` combos from scratch.
 - **Dark mode**: never add a `:root.theme-dark` block inside a component-scoped `<style>`. Use
@@ -350,6 +437,10 @@ These rules apply when writing or generating component, style, or page code.
 - **Per-component `:root.theme-dark` rules** — all dark remaps live in `theme.css`.
 - **Ad-hoc heading CSS** — no `h2 { font-size: 1.5rem; font-weight: 500; }` rules scattered
   across component files. Use the typography utilities.
+- **A color in a diagram** — no `fillcolor`, `color`, `fontcolor` or `bgcolor` in a Graphviz
+  fence, and no hex in a diagram component. Name a role. `pnpm check:diagram-tokens` fails CI on
+  this, because a hardcoded diagram color renders fine in light mode and wrong in dark, which is
+  how 63 of them accumulated without anyone noticing.
 
 ### Self-correction list
 
@@ -363,6 +454,8 @@ If you generate any of the following, **fix it immediately**:
 - `:root.theme-dark { … }` inside a `<style>` block in a component → move the logic to a semantic
   token in `theme.css`.
 - `font-size: 1.5rem; font-weight: 500;` heading ad-hoc combo → use `heading-section` class.
+- A color attribute inside a `` ```dot `` fence → replace it with the role that says what the
+  element *is* (see the role table under "Diagrams").
 
 No exceptions unless the user explicitly overrides.
 
diff --git a/plugins/remark-graphviz.test.ts b/plugins/remark-graphviz.test.ts
index ce61406cee..4b49be0411 100644
--- a/plugins/remark-graphviz.test.ts
+++ b/plugins/remark-graphviz.test.ts
@@ -34,7 +34,7 @@ describe('remarkGraphvizPlugin', () => {
     expect(svg).toMatch(/^<svg[^>]*class="dg queue"/);
   });
 
-  it('leaves no colour in the output at all', async () => {
+  it('leaves no color in the output at all', async () => {
     const svg = await render(`digraph {
       node [style=filled, fillcolor="#347D39", fontcolor="white"];
       subgraph cluster_b { fillcolor="#1CB893"; style="rounded,filled"; label="Batch"; A; }
@@ -47,7 +47,7 @@ describe('remarkGraphvizPlugin', () => {
 
   it('passes the role a fence names straight through to the SVG', async () => {
     // The whole system rests on this: Graphviz copies `class` verbatim, so the
-    // plugin never has to resolve a colour and CSS can do it at paint time.
+    // plugin never has to resolve a color and CSS can do it at paint time.
     const svg = await render(`digraph {
       A [class="failed"];
       subgraph cluster_b { class="batch"; label="Batch"; A; }
@@ -108,7 +108,7 @@ describe('remarkGraphvizPlugin', () => {
     expect(square).toMatch(/<g id="node1"[^>]*>\s*<title>A<\/title>\s*<polygon/);
   });
 
-  it('strips the alpha Graphviz emits alongside a colour', async () => {
+  it('strips the alpha Graphviz emits alongside a color', async () => {
     const svg = await render(`digraph {
       node [style=filled];
       A [fillcolor="#347d3980"];
diff --git a/plugins/remark-graphviz.ts b/plugins/remark-graphviz.ts
index 97922f25e7..40dd08b100 100644
--- a/plugins/remark-graphviz.ts
+++ b/plugins/remark-graphviz.ts
@@ -6,12 +6,12 @@ import { CONTINUE, visit } from 'unist-util-visit';
 import { finishDiagramSvg } from '../src/util/diagramSvg';
 
 /**
- * Render `dot` / `circo` / `neato` fences to inline SVG, and hand every colour
+ * Render `dot` / `circo` / `neato` fences to inline SVG, and hand every color
  * decision to CSS.
  *
  * Graphviz supports a `class` attribute on graphs, nodes, edges and clusters
  * and copies it verbatim into the SVG (`class="node queued"`). So this plugin
- * never resolves a colour and never names a role: the fence names them, and
+ * never resolves a color and never names a role: the fence names them, and
  * the plugin only injects shape and spacing defaults, drops the opaque canvas
  * and strips the inline paint, so the `.dg` rules in `index.css` resolve
  * surface, border and label at paint time from the role accents in
@@ -21,7 +21,7 @@ import { finishDiagramSvg } from '../src/util/diagramSvg';
  *
  * The one class the rendering side still adds is `plain`, in
  * `finishDiagramSvg` — a shape fact (this element is a caption, not a box),
- * not a colour.
+ * not a color.
  */
 
 const viz = await instance();
@@ -55,7 +55,7 @@ const BASE = `
 
 /**
  * Diagram kinds. A fence opts into one through its class — ```dot class="queue"
- * — and it sets layout, never colour. A fence that names none gets BASE alone
+ * — and it sets layout, never color. A fence that names none gets BASE alone
  * and lays itself out.
  */
 const KINDS: Record<string, string> = {
diff --git a/scripts/check-diagram-tokens.mjs b/scripts/check-diagram-tokens.mjs
index 4047c02132..49e0155e67 100644
--- a/scripts/check-diagram-tokens.mjs
+++ b/scripts/check-diagram-tokens.mjs
@@ -1,15 +1,15 @@
 #!/usr/bin/env node
 /**
- * Scan the docs for diagrams that name a colour.
+ * Scan the docs for diagrams that name a color.
  *
  * A diagram names a *role* — `queued`, `merged`, `failed`, `external` — and the
  * page resolves it from the design tokens at paint time, so the diagram themes
  * itself and every page uses one palette. That worked until it didn't: before
- * this rule existed, 63 distinct colours had accumulated across 18 pages in four
+ * this rule existed, 63 distinct colors had accumulated across 18 pages in four
  * unrelated dialects, none of which the token system could reach and none of
  * which adapted to dark mode.
  *
- * Nothing about a hardcoded colour fails a build on its own — the diagram
+ * Nothing about a hardcoded color fails a build on its own — the diagram
  * renders, it just renders wrong on half the site — so this is the thing that
  * keeps it from happening again. See DESIGN.md "Diagrams" for the roles.
  *
@@ -47,16 +47,16 @@ const SCANNED_EXTENSIONS = ['.mdx', '.md'];
 export const RULES = [
   {
     id: 'graphviz-color-attr',
-    label: 'Graphviz colour attribute',
+    label: 'Graphviz color attribute',
     // `(?<![\w-])` so `fillcolor` is reported once, as itself, rather than also
     // matching the bare `color` inside it. `none` and `transparent` are not
-    // colours — they are the absence of one, which is shape work, not styling,
+    // colors — they are the absence of one, which is shape work, not styling,
     // and the way a diagram says "draw no canvas" or "draw no border".
     re: /(?<![\w-])(?:bg|fill|font|pen|label)?color\s*=\s*(?!"?(?:none|transparent)\b)/gi,
   },
   {
     id: 'diagram-hex',
-    label: 'hardcoded colour',
+    label: 'hardcoded color',
     re: /#[0-9a-f]{6}\b/gi,
   },
 ];
@@ -88,7 +88,7 @@ function allowedOnLine(lines, index) {
 /**
  * The 1-based line numbers that belong to a diagram: everything inside a
  * Graphviz fence, plus its attribute line. Everything else in a docs page is
- * prose and code samples, where a colour is ordinary content — a CSS example,
+ * prose and code samples, where a color is ordinary content — a CSS example,
  * a screenshot description, a config value.
  */
 function diagramLines(text) {
@@ -169,21 +169,21 @@ function main(argv) {
     return findings.length === 0 ? 0 : 1;
   }
 
-  console.log(`Scanned ${scanned} file(s) for diagrams that name a colour.`);
+  console.log(`Scanned ${scanned} file(s) for diagrams that name a color.`);
   if (findings.length === 0) {
-    console.log('Every diagram resolves its colours from tokens.');
+    console.log('Every diagram resolves its colors from tokens.');
     return 0;
   }
-  console.error(`\n${findings.length} diagram colour(s) written by hand:\n`);
+  console.error(`\n${findings.length} diagram color(s) written by hand:\n`);
   for (const f of findings) {
     console.error(`  ${f.file}:${f.line} — ${f.label} — ${f.match}`);
   }
   console.error(
-    '\nName a role instead, and let the page resolve the colour:\n' +
+    '\nName a role instead, and let the page resolve the color:\n' +
       '  PR1 [class="queued"];   not   PR1 [fillcolor="#347D39"];\n' +
       'The roles are queued, pending, merged, failed, config, mergify,\n' +
       'datastore, external, batch, muted and chrome; `plain` marks a caption\n' +
-      'rather than a box. See DESIGN.md "Diagrams". If a colour is genuinely\n' +
+      'rather than a box. See DESIGN.md "Diagrams". If a color is genuinely\n' +
       'the subject rather than the styling, allow it on the line above:\n' +
       '  // diagram-tokens: allow <rule-id>[, <rule-id>...] — why'
   );
diff --git a/scripts/check-diagram-tokens.test.mjs b/scripts/check-diagram-tokens.test.mjs
index f3dbf7d86f..896ec4393e 100644
--- a/scripts/check-diagram-tokens.test.mjs
+++ b/scripts/check-diagram-tokens.test.mjs
@@ -5,7 +5,7 @@ const fence = (body) => ['```dot class="queue"', body, '```'].join('\n');
 const rulesOf = (text, opts) => scanText(text, opts).map((f) => f.rule);
 
 describe('scanText', () => {
-  it('catches a colour written inside a diagram', () => {
+  it('catches a color written inside a diagram', () => {
     expect(rulesOf(fence('A [fillcolor="#347D39"];'))).toEqual([
       'graphviz-color-attr',
       'diagram-hex',
@@ -18,7 +18,7 @@ describe('scanText', () => {
       'graphviz-color-attr',
       'diagram-hex',
     ]);
-    // A named colour carries no hex, and is just as unreachable by the tokens.
+    // A named color carries no hex, and is just as unreachable by the tokens.
     expect(rulesOf(fence('A [fontcolor=white];'))).toEqual(['graphviz-color-attr']);
   });
 
@@ -39,7 +39,7 @@ describe('scanText', () => {
     ).toEqual([]);
   });
 
-  it('treats none and transparent as shape, not colour', () => {
+  it('treats none and transparent as shape, not color', () => {
     // These say "draw nothing", which is the one thing a role cannot express.
     expect(rulesOf(fence('graph [bgcolor="transparent"];'))).toEqual([]);
     expect(rulesOf(fence('A [color=none];'))).toEqual([]);
@@ -53,10 +53,10 @@ describe('scanText', () => {
     expect(rulesOf(fence('PR1 [label="PR #101\\nScopes: frontend"];'))).toEqual([]);
   });
 
-  it('ignores colours outside a diagram', () => {
-    // A docs page is mostly prose and code samples, where a colour is content.
+  it('ignores colors outside a diagram', () => {
+    // A docs page is mostly prose and code samples, where a color is content.
     const page = [
-      'Set the badge colour with `color="#347D39"` in your config.',
+      'Set the badge color with `color="#347D39"` in your config.',
       '',
       '```css',
       '.badge { color: #347d39; }',
@@ -78,7 +78,7 @@ describe('scanText', () => {
   it('honours an allow directive on the line above', () => {
     expect(
       rulesOf(
-        fence(`// diagram-tokens: allow diagram-hex — this diagram is about the colour
+        fence(`// diagram-tokens: allow diagram-hex — this diagram is about the color
 A [label="#347D39"];`)
       )
     ).toEqual([]);
@@ -86,7 +86,7 @@ A [label="#347D39"];`)
 });
 
 describe('the docs themselves', () => {
-  it('has no diagram that names a colour', () => {
+  it('has no diagram that names a color', () => {
     const findings = [...iterFiles(['src/content/docs', 'src/components'])].flatMap(scanFile);
     expect(findings).toEqual([]);
   });
diff --git a/src/components/GitGraph.astro b/src/components/GitGraph.astro
index 30151bb54c..9a5803910f 100644
--- a/src/components/GitGraph.astro
+++ b/src/components/GitGraph.astro
@@ -6,7 +6,7 @@ import { type DiagramRole, finishDiagramSvg } from '~/util/diagramSvg';
 /**
  * Commit graphs, in two modes.
  *
- * Neither mode names a colour. A call site names a *role* — the same vocabulary
+ * Neither mode names a color. A call site names a *role* — the same vocabulary
  * the `dot` fences use — and `.dg` in index.css resolves it at paint time, so
  * this component and the fences share one palette and one dark mode. See
  * DESIGN.md for the roles.
diff --git a/src/styles/index.css b/src/styles/index.css
index 0514ed93a7..fcaa158b04 100644
--- a/src/styles/index.css
+++ b/src/styles/index.css
@@ -805,10 +805,10 @@ html {
 
 /* ── Diagrams ────────────────────────────────────────────────────────────────
    Graphviz copies a `class` attribute straight into the SVG, so the plugin
-   renders shape and layout and nothing else; every colour is resolved here, at
+   renders shape and layout and nothing else; every color is resolved here, at
    paint time, from the role tokens in theme.css. That is what makes dark mode
    free — the same `:root.theme-dark` block as every other surface, no second
-   render — and what lets a diagram be written without naming a colour.
+   render — and what lets a diagram be written without naming a color.
 
    Each role carries one accent. These lines turn it into the surface, the
    border and the label, by mixing it against the page background. A role that
diff --git a/src/styles/theme.css b/src/styles/theme.css
index 30c941a5df..e6d6c65864 100644
--- a/src/styles/theme.css
+++ b/src/styles/theme.css
@@ -104,11 +104,11 @@
   --theme-border-color: var(--theme-border);
   --theme-shade-subtle: var(--color-gray-200);
 
-  /* Diagrams — role accents. A diagram names a role, never a colour; this is
-     the whole palette, one line per role. The paint rules that consume them
-     are the `.dg` block in index.css. */
+  /* Diagrams — role accents. A diagram names a role, never a color; this is
+     the whole palette, one line per role. The paint rules that consume them are
+     the `.dg` block in index.css. See DESIGN.md "Diagrams". */
   --dg-a-chrome: var(--color-gray-700); /* edges, arrowheads, captions */
-  --dg-a-muted: var(--color-gray-400); /* skipped, dashed, de-emphasised */
+  --dg-a-muted: var(--color-gray-400); /* skipped, dashed, de-emphasized */
   --dg-a-batch: var(--color-gray-500); /* a grouping container */
   --dg-a-external: var(--color-gray-500); /* GitHub, CI — not us */
   --dg-a-queued: var(--color-teal-700); /* in the queue, waiting its turn */
@@ -160,7 +160,7 @@
 
   /* Navbar — content surface (white in light) so it reads as a distinct
      elevated zone over the body bg. Dark mode aliases --theme-bg-content
-     to --theme-bg, so this is unchanged from the previous behaviour there. */
+     to --theme-bg, so this is unchanged from the previous behavior there. */
   --theme-navbar-bg: var(--theme-bg-content);
 
   /* Glow */
diff --git a/src/util/diagramSvg.ts b/src/util/diagramSvg.ts
index e52c6429ef..f654cccff9 100644
--- a/src/util/diagramSvg.ts
+++ b/src/util/diagramSvg.ts
@@ -6,18 +6,18 @@ type CheerioAPI = ReturnType<typeof load>;
  * Shared post-processing for the two Graphviz surfaces on the site: the `dot`
  * fences (`plugins/remark-graphviz.ts`) and the `<GitGraph>` component.
  *
- * Both used to strip the canvas and rewrite colours their own way, with their
+ * Both used to strip the canvas and rewrite colors their own way, with their
  * own list of hex strings, which is how they drifted apart. Neither handles
- * colour now: this drops the canvas, strips the inline paint, and leaves the
+ * color now: this drops the canvas, strips the inline paint, and leaves the
  * `class` attribute Graphviz copied out of the source for the `.dg` rules in
- * `index.css` to resolve at paint time.
+ * `index.css` to resolve at paint time. See DESIGN.md "Diagrams".
  */
 
 /** The kinds of element `.dg` paints, and the Graphviz group class for each. */
 export type DiagramKind = 'node' | 'edge' | 'cluster';
 
 /**
- * Roles a diagram may name. `plain` is a shape marker rather than a colour: it
+ * Roles a diagram may name. `plain` is a shape marker rather than a color: it
  * says the element is a caption, not a box.
  *
  * Every role here has a `.dg .<role>` rule in `index.css` pointing at a