diff --git a/src/__testing__/theme.test.ts b/src/__testing__/theme.test.ts
index 305fecc00..c3e2ec9b0 100644
--- a/src/__testing__/theme.test.ts
+++ b/src/__testing__/theme.test.ts
@@ -3,6 +3,7 @@ import {
   readableTextColor,
   SistentDefaultPrimitivePaletteLight
 } from '../theme';
+import { MuiButton } from '../theme/components/button.modifier';
 
 const LIGHT_INK = SistentDefaultPrimitivePaletteLight.primaryInverted; // charcoal[100] ≈ near-white
 const DARK_INK = SistentDefaultPrimitivePaletteLight.foreground; // charcoal[10] ≈ near-black
@@ -28,7 +29,7 @@ describe('readableTextColor', () => {
 describe('createCustomTheme contrast derivation', () => {
   it('derives readable body text when a brand omits the contrast tokens', () => {
     // Only base colors provided; background is dark. The derived foreground
-    // (text.default) must be light so text stays readable — not the Layer5
+    // (text.default) must be light so text stays readable - not the Layer5
     // default dark ink.
     const theme = createCustomTheme('light', {
       navigationBar: '#101010',
@@ -57,7 +58,7 @@ describe('createCustomTheme contrast derivation', () => {
 
   it('keeps default contrast tokens for base colors the caller did not customize', () => {
     // Only `background` is overridden, so its contrast token (foreground)
-    // re-derives — but `primary` is untouched, so its contrast token must
+    // re-derives - but `primary` is untouched, so its contrast token must
     // keep the default ink rather than silently re-deriving from KEPPEL.
     const theme = createCustomTheme('light', { background: '#000000' });
 
@@ -81,3 +82,65 @@ describe('createCustomTheme contrast derivation', () => {
     expect(theme.palette.mode).toBe('light');
   });
 });
+
+describe('MuiButton contained honors the semantic color prop', () => {
+  // The MuiButton override function computes styles from the active theme.
+  // Call it directly (MUI would call it as `root({ theme, ownerState })`) so the
+  // assertions read the resolved backgroundColor per contained variant.
+  const rootStyles = (mode: 'light' | 'dark') => {
+    const theme = createCustomTheme(mode);
+    const root = MuiButton.styleOverrides!.root as (arg: { theme: typeof theme }) => Record<
+      string,
+      { color?: string; backgroundColor?: string; ['&:hover']?: { backgroundColor?: string } }
+    >;
+    return { theme, styles: root({ theme }) };
+  };
+
+  it('paints color="error" contained buttons with the error background, not the brand (keppel)', () => {
+    const { theme, styles } = rootStyles('light');
+    const contained = styles['&.MuiButton-contained'];
+    const containedError = styles['&.MuiButton-containedError'];
+
+    expect(containedError).toBeDefined();
+    // Uses the semantic error background...
+    expect(containedError.backgroundColor).toBe(theme.palette.background.error.default);
+    expect(containedError['&:hover']?.backgroundColor).toBe(
+      theme.palette.background.error.hover
+    );
+    // ...and is NOT the brand background that `&.MuiButton-contained` sets - the
+    // regression that made `<Button variant="contained" color="error">` green.
+    expect(containedError.backgroundColor).not.toBe(contained.backgroundColor);
+  });
+
+  it('also honors success, warning, and info contained buttons', () => {
+    const { theme, styles } = rootStyles('light');
+    expect(styles['&.MuiButton-containedSuccess'].backgroundColor).toBe(
+      theme.palette.background.success.default
+    );
+    expect(styles['&.MuiButton-containedWarning'].backgroundColor).toBe(
+      theme.palette.background.warning.default
+    );
+    expect(styles['&.MuiButton-containedInfo'].backgroundColor).toBe(
+      theme.palette.background.info.default
+    );
+  });
+
+  it('derives a readable (WCAG) label colour per background, not a hardcoded white', () => {
+    const { theme, styles } = rootStyles('light');
+    // The light yellow warning background must get dark text, not an unreadable
+    // white one - getContrastText picks per luminance.
+    expect(styles['&.MuiButton-containedWarning'].color).toBe(
+      theme.palette.getContrastText(theme.palette.background.warning.default)
+    );
+    expect(styles['&.MuiButton-containedError'].color).toBe(
+      theme.palette.getContrastText(theme.palette.background.error.default)
+    );
+  });
+
+  it('resolves the error background in dark mode too', () => {
+    const { theme, styles } = rootStyles('dark');
+    expect(styles['&.MuiButton-containedError'].backgroundColor).toBe(
+      theme.palette.background.error.default
+    );
+  });
+});
diff --git a/src/theme/components/button.modifier.ts b/src/theme/components/button.modifier.ts
index eb85adb8d..e8db2524d 100644
--- a/src/theme/components/button.modifier.ts
+++ b/src/theme/components/button.modifier.ts
@@ -5,7 +5,7 @@ export const MuiButton: Components<Theme>['MuiButton'] = {
     root: ({ theme }) => {
       const {
         palette: {
-          background: { brand, hover },
+          background: { brand, hover, error, success, warning, info },
           text: { disabled, constant, neutral: TextNeutral },
           border: { neutral }
         },
@@ -22,6 +22,49 @@ export const MuiButton: Components<Theme>['MuiButton'] = {
             backgroundColor: brand?.hover
           }
         },
+        // Contained buttons default to the brand colour above. Honour the
+        // semantic `color` prop so error / success / warning / info actions are
+        // not silently painted brand (keppel) - the long-standing gap that made
+        // `<Button variant="contained" color="error">` render green. Text colour
+        // is derived per-background with getContrastText so a light background
+        // (notably the yellow warning) keeps a readable dark label instead of an
+        // unreadable white one (WCAG). These rules share the
+        // `&.MuiButton-contained` selector's (0,0,2,0) specificity and follow it
+        // in source order, so a semantic-colour button (which carries BOTH
+        // classes) resolves to its colour; the higher-specificity `.Mui-disabled`
+        // rules below still win when disabled.
+        '&.MuiButton-containedError': {
+          color: error?.default ? theme.palette.getContrastText(error.default) : constant?.white,
+          backgroundColor: error?.default,
+          '&:hover': {
+            backgroundColor: error?.hover
+          }
+        },
+        '&.MuiButton-containedSuccess': {
+          color: success?.default
+            ? theme.palette.getContrastText(success.default)
+            : constant?.white,
+          backgroundColor: success?.default,
+          '&:hover': {
+            backgroundColor: success?.hover
+          }
+        },
+        '&.MuiButton-containedWarning': {
+          color: warning?.default
+            ? theme.palette.getContrastText(warning.default)
+            : constant?.white,
+          backgroundColor: warning?.default,
+          '&:hover': {
+            backgroundColor: warning?.hover
+          }
+        },
+        '&.MuiButton-containedInfo': {
+          color: info?.default ? theme.palette.getContrastText(info.default) : constant?.white,
+          backgroundColor: info?.default,
+          '&:hover': {
+            backgroundColor: info?.hover
+          }
+        },
         '&.MuiButton-outlined': {
           border: `1px solid ${neutral?.default}`,
           '&:hover': {