diff --git a/CHANGELOG.md b/CHANGELOG.md
index ac6bb28..29a07a4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+### Fixed
+
+- **A selected tab underlined itself in the colour of the rule it sits on, and the first fix put it one brand shade off.** The indicator used `border-color-border`, the same token as the tab list's own bottom rule, so a selection marked itself with a thicker length of the very line under it and read as a grey smudge. Replacing it with a bare `selected:border-primary` fixed the smudge and introduced a subtler wrong: there is no `border-color-primary` alias, so the alias layer passes the token through untouched and wind's border parser defaults the missing shade to 500, while every other brand surface resolves `bg-primary` to primary-600 in light mode. An active tab therefore underlined in primary-500 beside a primary-600 button, and the bare token carried no `dark:` half at all, contrary to this project's own widget rules. Now `selected:border-primary-600 dark:selected:border-primary-500`, asserted against the colour wind itself resolves for those shades in both modes rather than against a hardcoded hex. (`lib/src/ui/components/tabs/tabs.recipe.dart`, `test/ui/components/tabs/tabs_test.dart`)
+
 ## [0.0.1-alpha.20] - 2026-08-17
 
 ### Fixed
diff --git a/lib/src/ui/components/tabs/tabs.recipe.dart b/lib/src/ui/components/tabs/tabs.recipe.dart
index 5ca021d..c38523e 100644
--- a/lib/src/ui/components/tabs/tabs.recipe.dart
+++ b/lib/src/ui/components/tabs/tabs.recipe.dart
@@ -17,8 +17,12 @@ Map<String, String> tabsRecipe({
   const recipe = WindSlotRecipe(
     slots: {
       'list': 'flex flex-row border-b border-color-border',
+      // The selected underline is the BRAND colour, not `border-color-border`:
+      // that is the same token as the `list` rule the tabs sit on, so an active
+      // tab used to mark itself with a thicker length of the very line it was
+      // sitting on. It read as a grey smudge rather than a selection.
       'tab':
-          'px-4 py-2 text-sm font-medium text-fg-muted cursor-pointer selected:text-fg selected:border-b-2 selected:border-color-border',
+          'px-4 py-2 text-sm font-medium text-fg-muted cursor-pointer selected:text-fg selected:border-b-2 selected:border-primary-600 dark:selected:border-primary-500',
       'panel': 'pt-4',
     },
   );
diff --git a/test/ui/components/tabs/tabs_test.dart b/test/ui/components/tabs/tabs_test.dart
index 8d66d43..0ca0dd5 100644
--- a/test/ui/components/tabs/tabs_test.dart
+++ b/test/ui/components/tabs/tabs_test.dart
@@ -73,6 +73,96 @@ void main() {
       expect(find.text('Panel 2'), findsNothing);
     });
 
+    testWidgets(
+        'the selected tab does not underline itself in the rule '
+        'colour', (tester) async {
+      // The indicator used `border-color-border`, the same token as the rule
+      // the tab list sits on, so a selected tab marked itself with a thicker
+      // length of the very line under it and read as a grey smudge. Asserted on
+      // the rendered colours rather than the className: the className was
+      // perfectly valid, it just resolved to the background it was drawn over.
+      await tester.pumpWidget(
+        wrap(
+          MSTabs(
+            tabs: const ['Tab 1', 'Tab 2'],
+            selectedIndex: 0,
+            onChanged: (_) {},
+            panelBuilder: (i) => Text('Panel $i'),
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      final Set<Color> bottomBorders = tester
+          .widgetList<DecoratedBox>(find.byType(DecoratedBox))
+          .map((box) => box.decoration)
+          .whereType<BoxDecoration>()
+          .where((decoration) => (decoration.border?.bottom.width ?? 0) > 0)
+          .map((decoration) => decoration.border!.bottom.color)
+          .toSet();
+
+      expect(
+        bottomBorders.length,
+        greaterThan(1),
+        reason: 'the active indicator and the list rule cannot be the same '
+            'colour, or the selection is invisible',
+      );
+
+      // And it is the BRAND, not merely something else. The assertion above
+      // stays green if the token becomes `border-red-500`, so on its own it pins
+      // the indicator away from the rule without pinning it to anything. A bare
+      // `border-primary` is what made this necessary: there is no
+      // `border-color-primary` alias, so the alias layer passes it through and
+      // wind's border parser defaults the missing shade to 500, while every
+      // other brand surface resolves `bg-primary` to primary-600 in light mode.
+      // One shade off, next to a button that is not.
+      expect(
+        bottomBorders,
+        contains(_brandShade(600)),
+        reason: 'the indicator must be the same brand shade as bg-primary',
+      );
+    });
+
+    testWidgets('the indicator tracks the brand in dark mode too', (
+      tester,
+    ) async {
+      // `.claude/rules/widgets.md`: always pair light and dark. The bare token
+      // this replaced had no `dark:` half at all, so dark mode inherited the
+      // light shade.
+      // `syncWithSystem: false` so the brightness set here survives: a syncing
+      // theme is forced to the test platform's brightness on startup, which is
+      // light, and the whole case would then assert light twice.
+      await tester.pumpWidget(
+        MaterialApp(
+          home: WindTheme(
+            data: WindThemeData(
+              brightness: Brightness.dark,
+              syncWithSystem: false,
+            ),
+            child: Scaffold(
+              body: MSTabs(
+                tabs: const ['Tab 1', 'Tab 2'],
+                selectedIndex: 0,
+                onChanged: (_) {},
+                panelBuilder: (i) => Text('Panel $i'),
+              ),
+            ),
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      final Set<Color> bottomBorders = tester
+          .widgetList<DecoratedBox>(find.byType(DecoratedBox))
+          .map((box) => box.decoration)
+          .whereType<BoxDecoration>()
+          .where((decoration) => (decoration.border?.bottom.width ?? 0) > 0)
+          .map((decoration) => decoration.border!.bottom.color)
+          .toSet();
+
+      expect(bottomBorders, contains(_brandShade(500)));
+    });
+
     testWidgets('calls onChanged when a tab is tapped', (tester) async {
       int? changed;
       await tester.pumpWidget(
@@ -112,3 +202,16 @@ void main() {
     });
   });
 }
+
+/// The brand colour wind resolves `primary-<shade>` to, read from wind itself
+/// rather than hardcoded.
+///
+/// Hardcoding a hex here would pin the test to Tailwind's palette rather than to
+/// the theme, and the first guess (Material's `Colors.blue.shade600`) was a
+/// different colour entirely: wind's default primary is Tailwind blue, so the
+/// indicator renders #2563EB and Material's 600 is #1E88E5.
+Color _brandShade(int shade) {
+  final WindThemeData theme = WindThemeData();
+
+  return theme.colors['primary']![shade]!;
+}