diff --git a/.claude/rules/ui.md b/.claude/rules/ui.md
index 3914dbe3..8b362d79 100644
--- a/.claude/rules/ui.md
+++ b/.claude/rules/ui.md
@@ -25,6 +25,8 @@ The UI has 4 main tabs: **Exchanges**, **Dashboard**, **Runtime**, **Deployments
 - `ui/src/auth/auth-store.ts` — Zustand: accessToken, user, roles, login/logout
 - `ui/src/api/environment-store.ts` — Zustand: selected environment (localStorage)
 - `ui/src/components/ContentTabs.tsx` — main tab switcher
+- `ui/src/components/EnvironmentSwitcherButton.tsx` + `EnvironmentSwitcherModal.tsx` — explicit env picker (button in TopBar; DS `Modal`-based list). Replaces the retired `EnvironmentSelector` (All-Envs dropdown). When `envRecords.length > 0` and the stored `selectedEnv` no longer matches any env, `LayoutShell` opens the modal in `forced` mode (non-dismissible). Switcher pulls env records from `useEnvironments()` (admin endpoint; readable by VIEWER+).
+- `ui/src/components/env-colors.ts` + `ui/src/styles/env-colors.css` — 8-swatch preset palette for the per-environment color indicator. Tokens `--env-color-slate/red/amber/green/teal/blue/purple/pink` are defined for both light and dark themes. `envColorVar(name)` falls back to `slate` for unknown values. `LayoutShell` renders a 3px fixed top bar in the current env's color (z-index 900, below DS modals).
 - `ui/src/components/ExecutionDiagram/` — interactive trace view (canvas)
 - `ui/src/components/ProcessDiagram/` — ELK-rendered route diagram
 - `ui/src/hooks/useScope.ts` — TabKey type, scope inference
diff --git a/ui/src/api/queries/admin/environments.ts b/ui/src/api/queries/admin/environments.ts
index c32888c0..e3c8310e 100644
--- a/ui/src/api/queries/admin/environments.ts
+++ b/ui/src/api/queries/admin/environments.ts
@@ -9,6 +9,7 @@ export interface Environment {
   enabled: boolean;
   defaultContainerConfig: Record<string, unknown>;
   jarRetentionCount: number | null;
+  color: string;
   createdAt: string;
 }
 
@@ -22,6 +23,7 @@ export interface UpdateEnvironmentRequest {
   displayName: string;
   production: boolean;
   enabled: boolean;
+  color?: string;
 }
 
 export function useEnvironments() {
diff --git a/ui/src/components/EnvironmentSelector.module.css b/ui/src/components/EnvironmentSelector.module.css
deleted file mode 100644
index 2d3518ac..00000000
--- a/ui/src/components/EnvironmentSelector.module.css
+++ /dev/null
@@ -1,4 +0,0 @@
-/* Layout wrapper — DS Select handles its own appearance */
-.select {
-  min-width: 100px;
-}
diff --git a/ui/src/components/EnvironmentSelector.tsx b/ui/src/components/EnvironmentSelector.tsx
deleted file mode 100644
index 2230e654..00000000
--- a/ui/src/components/EnvironmentSelector.tsx
+++ /dev/null
@@ -1,30 +0,0 @@
-import { useMemo } from 'react';
-import { Select } from '@cameleer/design-system';
-import styles from './EnvironmentSelector.module.css';
-
-interface EnvironmentSelectorProps {
-  environments: string[];
-  value: string | undefined;
-  onChange: (env: string | undefined) => void;
-}
-
-export function EnvironmentSelector({ environments, value, onChange }: EnvironmentSelectorProps) {
-  if (environments.length === 0) return null;
-
-  const options = useMemo(
-    () => [
-      { value: '', label: 'All Envs' },
-      ...environments.map((env) => ({ value: env, label: env })),
-    ],
-    [environments],
-  );
-
-  return (
-    <Select
-      className={styles.select}
-      options={options}
-      value={value ?? ''}
-      onChange={(e) => onChange(e.target.value || undefined)}
-    />
-  );
-}
diff --git a/ui/src/components/EnvironmentSwitcherButton.module.css b/ui/src/components/EnvironmentSwitcherButton.module.css
new file mode 100644
index 00000000..e8a0b29f
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherButton.module.css
@@ -0,0 +1,44 @@
+.button {
+    display: inline-flex;
+    align-items: center;
+    gap: 8px;
+    padding: 6px 10px;
+    height: 32px;
+    background: var(--bg-surface);
+    border: 1px solid var(--border-subtle);
+    border-radius: 6px;
+    color: var(--text-primary);
+    font-size: 13px;
+    font-weight: 500;
+    cursor: pointer;
+    transition: background 0.15s ease, border-color 0.15s ease;
+}
+
+.button:hover {
+    background: var(--bg-surface-hover, var(--bg-surface));
+    border-color: var(--border-strong, var(--border-subtle));
+}
+
+.button:focus-visible {
+    outline: 2px solid var(--accent);
+    outline-offset: 2px;
+}
+
+.dot {
+    width: 10px;
+    height: 10px;
+    border-radius: 50%;
+    flex-shrink: 0;
+}
+
+.name {
+    max-width: 160px;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.chevron {
+    color: var(--text-muted);
+    flex-shrink: 0;
+}
diff --git a/ui/src/components/EnvironmentSwitcherButton.test.tsx b/ui/src/components/EnvironmentSwitcherButton.test.tsx
new file mode 100644
index 00000000..f2d13109
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherButton.test.tsx
@@ -0,0 +1,57 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { EnvironmentSwitcherButton } from './EnvironmentSwitcherButton';
+import type { Environment } from '../api/queries/admin/environments';
+
+const envs: Environment[] = [
+  {
+    id: '11111111-1111-1111-1111-111111111111',
+    slug: 'dev',
+    displayName: 'Development',
+    production: false,
+    enabled: true,
+    defaultContainerConfig: {},
+    jarRetentionCount: 5,
+    color: 'amber',
+    createdAt: '2026-04-22T00:00:00Z',
+  },
+  {
+    id: '22222222-2222-2222-2222-222222222222',
+    slug: 'prod',
+    displayName: 'Production',
+    production: true,
+    enabled: true,
+    defaultContainerConfig: {},
+    jarRetentionCount: 10,
+    color: 'red',
+    createdAt: '2026-04-22T00:00:00Z',
+  },
+];
+
+describe('EnvironmentSwitcherButton', () => {
+  it('renders the selected env display name', () => {
+    render(<EnvironmentSwitcherButton envs={envs} value="dev" onClick={() => {}} />);
+    expect(screen.getByText('Development')).toBeInTheDocument();
+  });
+
+  it('shows placeholder text when no env is selected', () => {
+    render(<EnvironmentSwitcherButton envs={envs} value={undefined} onClick={() => {}} />);
+    expect(screen.getByText(/select environment/i)).toBeInTheDocument();
+  });
+
+  it('fires onClick when pressed', () => {
+    const onClick = vi.fn();
+    render(<EnvironmentSwitcherButton envs={envs} value="dev" onClick={onClick} />);
+    fireEvent.click(screen.getByRole('button', { name: /switch environment/i }));
+    expect(onClick).toHaveBeenCalledOnce();
+  });
+
+  it('paints the color dot with the env color CSS variable', () => {
+    const { container } = render(
+      <EnvironmentSwitcherButton envs={envs} value="prod" onClick={() => {}} />,
+    );
+    const dot = container.querySelector('span[aria-hidden]');
+    expect(dot).toBeTruthy();
+    expect((dot as HTMLElement).style.background).toContain('env-color-red');
+  });
+});
diff --git a/ui/src/components/EnvironmentSwitcherButton.tsx b/ui/src/components/EnvironmentSwitcherButton.tsx
new file mode 100644
index 00000000..e8df5b77
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherButton.tsx
@@ -0,0 +1,30 @@
+import { ChevronDown } from 'lucide-react';
+import type { Environment } from '../api/queries/admin/environments';
+import { envColorVar } from './env-colors';
+import styles from './EnvironmentSwitcherButton.module.css';
+
+interface EnvironmentSwitcherButtonProps {
+  envs: Environment[];
+  value: string | undefined;
+  onClick: () => void;
+}
+
+export function EnvironmentSwitcherButton({ envs, value, onClick }: EnvironmentSwitcherButtonProps) {
+  const current = envs.find((e) => e.slug === value);
+  const displayName = current?.displayName ?? value ?? 'Select environment';
+  const color = envColorVar(current?.color);
+
+  return (
+    <button
+      type="button"
+      className={styles.button}
+      onClick={onClick}
+      aria-label="Switch environment"
+      aria-haspopup="dialog"
+    >
+      <span className={styles.dot} style={{ background: color }} aria-hidden />
+      <span className={styles.name}>{displayName}</span>
+      <ChevronDown size={14} className={styles.chevron} aria-hidden />
+    </button>
+  );
+}
diff --git a/ui/src/components/EnvironmentSwitcherModal.module.css b/ui/src/components/EnvironmentSwitcherModal.module.css
new file mode 100644
index 00000000..13265577
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherModal.module.css
@@ -0,0 +1,82 @@
+.list {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+}
+
+.row {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    width: 100%;
+    padding: 10px 12px;
+    background: var(--bg-surface);
+    border: 1px solid var(--border-subtle);
+    border-radius: 6px;
+    color: var(--text-primary);
+    cursor: pointer;
+    text-align: left;
+    transition: background 0.15s ease, border-color 0.15s ease;
+}
+
+.row:hover {
+    background: var(--bg-surface-hover, var(--bg-elevated));
+    border-color: var(--border-strong, var(--border-subtle));
+}
+
+.row:focus-visible {
+    outline: 2px solid var(--accent);
+    outline-offset: 2px;
+}
+
+.rowSelected {
+    border-color: var(--accent);
+}
+
+.dot {
+    width: 12px;
+    height: 12px;
+    border-radius: 50%;
+    flex-shrink: 0;
+}
+
+.labels {
+    display: flex;
+    flex-direction: column;
+    flex: 1;
+    min-width: 0;
+}
+
+.name {
+    font-size: 13px;
+    font-weight: 500;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.slug {
+    color: var(--text-muted);
+    margin-top: 2px;
+}
+
+.badges {
+    display: flex;
+    gap: 4px;
+    flex-shrink: 0;
+}
+
+.check {
+    color: var(--accent);
+    flex-shrink: 0;
+}
+
+.empty {
+    padding: 20px;
+    text-align: center;
+    color: var(--text-muted);
+    font-size: 13px;
+}
diff --git a/ui/src/components/EnvironmentSwitcherModal.test.tsx b/ui/src/components/EnvironmentSwitcherModal.test.tsx
new file mode 100644
index 00000000..6ef02114
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherModal.test.tsx
@@ -0,0 +1,111 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { ThemeProvider } from '@cameleer/design-system';
+import type { ReactNode } from 'react';
+import { EnvironmentSwitcherModal } from './EnvironmentSwitcherModal';
+import type { Environment } from '../api/queries/admin/environments';
+
+function wrap(ui: ReactNode) {
+  return render(<ThemeProvider>{ui}</ThemeProvider>);
+}
+
+const envs: Environment[] = [
+  {
+    id: '11111111-1111-1111-1111-111111111111',
+    slug: 'dev',
+    displayName: 'Development',
+    production: false,
+    enabled: true,
+    defaultContainerConfig: {},
+    jarRetentionCount: 5,
+    color: 'amber',
+    createdAt: '2026-04-22T00:00:00Z',
+  },
+  {
+    id: '22222222-2222-2222-2222-222222222222',
+    slug: 'prod',
+    displayName: 'Production',
+    production: true,
+    enabled: true,
+    defaultContainerConfig: {},
+    jarRetentionCount: 10,
+    color: 'red',
+    createdAt: '2026-04-22T00:00:00Z',
+  },
+];
+
+describe('EnvironmentSwitcherModal', () => {
+  it('renders one row per env when open', () => {
+    wrap(
+      <EnvironmentSwitcherModal
+        open
+        onClose={() => {}}
+        envs={envs}
+        value="dev"
+        onPick={() => {}}
+      />,
+    );
+    expect(screen.getByText('Development')).toBeInTheDocument();
+    expect(screen.getByText('Production')).toBeInTheDocument();
+  });
+
+  it('calls onPick with slug when a row is clicked', () => {
+    const onPick = vi.fn();
+    wrap(
+      <EnvironmentSwitcherModal
+        open
+        onClose={() => {}}
+        envs={envs}
+        value="dev"
+        onPick={onPick}
+      />,
+    );
+    fireEvent.click(screen.getByRole('option', { name: /production/i }));
+    expect(onPick).toHaveBeenCalledWith('prod');
+  });
+
+  it('marks the current env with aria-selected', () => {
+    wrap(
+      <EnvironmentSwitcherModal
+        open
+        onClose={() => {}}
+        envs={envs}
+        value="dev"
+        onPick={() => {}}
+      />,
+    );
+    const selected = screen.getByRole('option', { name: /development/i });
+    expect(selected).toHaveAttribute('aria-selected', 'true');
+  });
+
+  it('renders empty state when no envs exist', () => {
+    wrap(
+      <EnvironmentSwitcherModal
+        open
+        onClose={() => {}}
+        envs={[]}
+        value={undefined}
+        onPick={() => {}}
+      />,
+    );
+    expect(screen.getByText(/no environments/i)).toBeInTheDocument();
+  });
+
+  it('forced mode swaps the title and ignores onClose', () => {
+    const onClose = vi.fn();
+    wrap(
+      <EnvironmentSwitcherModal
+        open
+        onClose={onClose}
+        envs={envs}
+        value={undefined}
+        onPick={() => {}}
+        forced
+      />,
+    );
+    expect(screen.getByText(/select an environment/i)).toBeInTheDocument();
+    // Simulate ESC — DS Modal forwards this to onClose, which we wrapped in a no-op.
+    fireEvent.keyDown(document, { key: 'Escape' });
+    expect(onClose).not.toHaveBeenCalled();
+  });
+});
diff --git a/ui/src/components/EnvironmentSwitcherModal.tsx b/ui/src/components/EnvironmentSwitcherModal.tsx
new file mode 100644
index 00000000..abb10a04
--- /dev/null
+++ b/ui/src/components/EnvironmentSwitcherModal.tsx
@@ -0,0 +1,66 @@
+import { Modal, Badge, MonoText } from '@cameleer/design-system';
+import { Check } from 'lucide-react';
+import type { Environment } from '../api/queries/admin/environments';
+import { envColorVar } from './env-colors';
+import styles from './EnvironmentSwitcherModal.module.css';
+
+interface EnvironmentSwitcherModalProps {
+  open: boolean;
+  onClose: () => void;
+  envs: Environment[];
+  value: string | undefined;
+  onPick: (slug: string) => void;
+  /** When true, ESC/backdrop do nothing — the user must pick one. */
+  forced?: boolean;
+}
+
+export function EnvironmentSwitcherModal({
+  open,
+  onClose,
+  envs,
+  value,
+  onPick,
+  forced = false,
+}: EnvironmentSwitcherModalProps) {
+  const title = forced ? 'Select an environment' : 'Switch environment';
+  const handleClose = forced ? () => { /* locked */ } : onClose;
+
+  return (
+    <Modal open={open} onClose={handleClose} title={title} size="sm">
+      {envs.length === 0 ? (
+        <div className={styles.empty}>
+          No environments — ask an admin to create one.
+        </div>
+      ) : (
+        <ul className={styles.list} role="listbox" aria-label="Environments">
+          {envs.map((env) => {
+            const selected = env.slug === value;
+            return (
+              <li key={env.id}>
+                <button
+                  type="button"
+                  className={`${styles.row} ${selected ? styles.rowSelected : ''}`}
+                  onClick={() => onPick(env.slug)}
+                  aria-selected={selected}
+                  role="option"
+                >
+                  <span className={styles.dot} style={{ background: envColorVar(env.color) }} aria-hidden />
+                  <div className={styles.labels}>
+                    <div className={styles.name}>{env.displayName}</div>
+                    <MonoText size="xs" className={styles.slug}>{env.slug}</MonoText>
+                  </div>
+                  <div className={styles.badges}>
+                    {env.production && <Badge label="PROD" color="error" />}
+                    {!env.production && <Badge label="NON-PROD" color="auto" />}
+                    {!env.enabled && <Badge label="DISABLED" color="warning" />}
+                  </div>
+                  {selected && <Check size={16} className={styles.check} aria-label="current" />}
+                </button>
+              </li>
+            );
+          })}
+        </ul>
+      )}
+    </Modal>
+  );
+}
diff --git a/ui/src/components/LayoutShell.tsx b/ui/src/components/LayoutShell.tsx
index 2775f6d1..833807fb 100644
--- a/ui/src/components/LayoutShell.tsx
+++ b/ui/src/components/LayoutShell.tsx
@@ -39,7 +39,9 @@ import { useEnvironmentStore } from '../api/environment-store';
 import { useState, useMemo, useCallback, useEffect, useRef, createElement } from 'react';
 import type { ReactNode } from 'react';
 import { ContentTabs } from './ContentTabs';
-import { EnvironmentSelector } from './EnvironmentSelector';
+import { EnvironmentSwitcherButton } from './EnvironmentSwitcherButton';
+import { EnvironmentSwitcherModal } from './EnvironmentSwitcherModal';
+import { envColorVar } from './env-colors';
 import { useScope } from '../hooks/useScope';
 import { formatDuration } from '../utils/format-utils';
 import {
@@ -428,6 +430,25 @@ function LayoutContent() {
     queryClient.invalidateQueries();
   }, [setSelectedEnvRaw, navigate, location.pathname, location.search, queryClient]);
 
+  // --- Env switcher modal -------------------------------------------
+  const [switcherOpen, setSwitcherOpen] = useState(false);
+
+  // Force-open the switcher when we have envs loaded but no valid selection.
+  // This replaces the old "All Envs" fallback: every session must pick one.
+  const selectionInvalid =
+    envRecords.length > 0 &&
+    (selectedEnv === undefined || !envRecords.some((e) => e.slug === selectedEnv));
+  const switcherForced = selectionInvalid;
+  useEffect(() => {
+    if (selectionInvalid) {
+      if (selectedEnv !== undefined) setSelectedEnvRaw(undefined);
+      setSwitcherOpen(true);
+    }
+  }, [selectionInvalid, selectedEnv, setSelectedEnvRaw]);
+
+  const currentEnvRecord = envRecords.find((e) => e.slug === selectedEnv);
+  const envBarColor = envColorVar(currentEnvRecord?.color);
+
   // --- Section open states ------------------------------------------
   const [appsOpen, setAppsOpen] = useState(() => (isAdminPage || isAlertsPage) ? false : readCollapsed(SK_APPS, true));
   const [adminOpen, setAdminOpen] = useState(() => isAdminPage ? true : readCollapsed(SK_ADMIN, false));
@@ -954,14 +975,38 @@ function LayoutContent() {
 
   return (
     <AppShell sidebar={sidebarElement}>
+      <div
+        aria-hidden
+        style={{
+          position: 'fixed',
+          top: 0,
+          left: 0,
+          right: 0,
+          height: 3,
+          background: envBarColor,
+          zIndex: 900,
+          pointerEvents: 'none',
+        }}
+      />
+      <EnvironmentSwitcherModal
+        open={switcherOpen}
+        onClose={() => setSwitcherOpen(false)}
+        envs={envRecords}
+        value={selectedEnv}
+        onPick={(slug) => {
+          setSelectedEnv(slug);
+          setSwitcherOpen(false);
+        }}
+        forced={switcherForced}
+      />
       <TopBar
         breadcrumb={breadcrumb}
         environment={
           <div style={{ display: 'flex', alignItems: 'center', gap: 12 }}>
-            <EnvironmentSelector
-              environments={environments}
+            <EnvironmentSwitcherButton
+              envs={envRecords}
               value={selectedEnv}
-              onChange={setSelectedEnv}
+              onClick={() => setSwitcherOpen(true)}
             />
             <NotificationBell />
           </div>
diff --git a/ui/src/components/env-colors.test.ts b/ui/src/components/env-colors.test.ts
new file mode 100644
index 00000000..93f3e03f
--- /dev/null
+++ b/ui/src/components/env-colors.test.ts
@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest';
+import { envColorVar, isEnvColor, ENV_COLORS, DEFAULT_ENV_COLOR } from './env-colors';
+
+describe('env-colors', () => {
+  it('maps known colors to their css variables', () => {
+    for (const c of ENV_COLORS) {
+      expect(envColorVar(c)).toBe(`var(--env-color-${c})`);
+    }
+  });
+
+  it('falls back to slate for unknown values', () => {
+    expect(envColorVar('neon')).toBe('var(--env-color-slate)');
+    expect(envColorVar(undefined)).toBe('var(--env-color-slate)');
+    expect(envColorVar(null)).toBe('var(--env-color-slate)');
+    expect(envColorVar('')).toBe('var(--env-color-slate)');
+  });
+
+  it('isEnvColor narrows types', () => {
+    expect(isEnvColor('amber')).toBe(true);
+    expect(isEnvColor('neon')).toBe(false);
+    expect(isEnvColor(undefined)).toBe(false);
+  });
+
+  it('default is slate', () => {
+    expect(DEFAULT_ENV_COLOR).toBe('slate');
+  });
+});
diff --git a/ui/src/components/env-colors.ts b/ui/src/components/env-colors.ts
new file mode 100644
index 00000000..82b2963d
--- /dev/null
+++ b/ui/src/components/env-colors.ts
@@ -0,0 +1,29 @@
+// Preset palette for per-environment color indicator. Mirrors the backend
+// EnvironmentColor enum. Add/remove entries here AND in:
+//   - backend V*__add_environment_color.sql CHECK constraint
+//   - cameleer-server-core/.../EnvironmentColor.java VALUES set
+//   - ui/src/styles/env-colors.css (token definitions)
+
+export const ENV_COLORS = [
+  'slate',
+  'red',
+  'amber',
+  'green',
+  'teal',
+  'blue',
+  'purple',
+  'pink',
+] as const;
+
+export type EnvColor = (typeof ENV_COLORS)[number];
+export const DEFAULT_ENV_COLOR: EnvColor = 'slate';
+
+export function isEnvColor(v: string | null | undefined): v is EnvColor {
+  return typeof v === 'string' && (ENV_COLORS as readonly string[]).includes(v);
+}
+
+// Safe for any string input — unknown/missing values resolve to the default
+// token. Use this whenever rendering an env's color; never concat directly.
+export function envColorVar(color: string | null | undefined): string {
+  return `var(--env-color-${isEnvColor(color) ? color : DEFAULT_ENV_COLOR})`;
+}
diff --git a/ui/src/main.tsx b/ui/src/main.tsx
index 2cc19807..3e09dd97 100644
--- a/ui/src/main.tsx
+++ b/ui/src/main.tsx
@@ -1,5 +1,6 @@
 import '@cameleer/design-system/style.css';
 import './index.css';
+import './styles/env-colors.css';
 import { StrictMode } from 'react';
 import { createRoot } from 'react-dom/client';
 import { RouterProvider } from 'react-router';
diff --git a/ui/src/pages/Admin/EnvironmentsPage.tsx b/ui/src/pages/Admin/EnvironmentsPage.tsx
index ef4cee2e..3fb70812 100644
--- a/ui/src/pages/Admin/EnvironmentsPage.tsx
+++ b/ui/src/pages/Admin/EnvironmentsPage.tsx
@@ -23,6 +23,8 @@ import {
   useUpdateJarRetention,
 } from '../../api/queries/admin/environments';
 import type { Environment } from '../../api/queries/admin/environments';
+import { ENV_COLORS, envColorVar, type EnvColor } from '../../components/env-colors';
+import { Check } from 'lucide-react';
 import { PageLoader } from '../../components/PageLoader';
 import styles from './UserManagement.module.css';
 import sectionStyles from '../../styles/section-card.module.css';
@@ -120,6 +122,7 @@ export default function EnvironmentsPage() {
         displayName: newName,
         production: selected.production,
         enabled: selected.enabled,
+        color: selected.color,
       });
       toast({ title: 'Environment renamed', variant: 'success' });
     } catch {
@@ -135,6 +138,7 @@ export default function EnvironmentsPage() {
         displayName: selected.displayName,
         production: value,
         enabled: selected.enabled,
+        color: selected.color,
       });
       toast({ title: value ? 'Marked as production' : 'Marked as non-production', variant: 'success' });
     } catch {
@@ -150,6 +154,7 @@ export default function EnvironmentsPage() {
         displayName: selected.displayName,
         production: selected.production,
         enabled: value,
+        color: selected.color,
       });
       toast({ title: value ? 'Environment enabled' : 'Environment disabled', variant: 'success' });
     } catch {
@@ -157,6 +162,22 @@ export default function EnvironmentsPage() {
     }
   }
 
+  async function handleColorChange(color: EnvColor) {
+    if (!selected || selected.color === color) return;
+    try {
+      await updateEnv.mutateAsync({
+        slug: selected.slug,
+        displayName: selected.displayName,
+        production: selected.production,
+        enabled: selected.enabled,
+        color,
+      });
+      toast({ title: 'Environment color updated', variant: 'success' });
+    } catch {
+      toast({ title: 'Failed to update color', variant: 'error', duration: 86_400_000 });
+    }
+  }
+
   if (isLoading) return <PageLoader />;
 
   return (
@@ -279,6 +300,44 @@ export default function EnvironmentsPage() {
                 </div>
               </div>
 
+              <div className={sectionStyles.section}>
+                <SectionHeader>Appearance</SectionHeader>
+                <p className={styles.inheritedNote}>
+                  This color is shown as a 3px bar across every page while this environment is active.
+                </p>
+                <div role="radiogroup" aria-label="Environment color" style={{ display: 'flex', gap: 10, flexWrap: 'wrap' }}>
+                  {ENV_COLORS.map((c) => {
+                    const isSelected = selected.color === c;
+                    return (
+                      <button
+                        key={c}
+                        type="button"
+                        role="radio"
+                        aria-checked={isSelected}
+                        aria-label={c}
+                        title={c}
+                        onClick={() => handleColorChange(c)}
+                        style={{
+                          width: 36,
+                          height: 36,
+                          borderRadius: '50%',
+                          background: envColorVar(c),
+                          border: isSelected ? '2px solid var(--text-primary)' : '1px solid var(--border-subtle)',
+                          cursor: 'pointer',
+                          display: 'inline-flex',
+                          alignItems: 'center',
+                          justifyContent: 'center',
+                          color: '#fff',
+                          padding: 0,
+                        }}
+                      >
+                        {isSelected && <Check size={16} aria-hidden />}
+                      </button>
+                    );
+                  })}
+                </div>
+              </div>
+
               <div className={sectionStyles.section}>
                 <SectionHeader>Status</SectionHeader>
                 <div className={styles.securitySection}>
diff --git a/ui/src/styles/env-colors.css b/ui/src/styles/env-colors.css
new file mode 100644
index 00000000..903d0e98
--- /dev/null
+++ b/ui/src/styles/env-colors.css
@@ -0,0 +1,34 @@
+/*
+ * Per-environment preset color tokens. Used by:
+ *   - LayoutShell's 3px top-bar indicator
+ *   - EnvironmentSwitcherButton / EnvironmentSwitcherModal (color dot)
+ *   - EnvironmentsPage "Appearance" swatch grid
+ *
+ * Light-mode and dark-mode values are tuned for WCAG-AA contrast against the
+ * respective surface tokens (`--bg-surface`, `--text-primary`).
+ *
+ * Storage: plain lowercase string ("slate", "red", ...). Unknown values fall
+ * back to `slate` via `envColorVar()`.
+ */
+
+:root {
+    --env-color-slate:  #94a3b8;
+    --env-color-red:    #ef4444;
+    --env-color-amber:  #f59e0b;
+    --env-color-green:  #10b981;
+    --env-color-teal:   #14b8a6;
+    --env-color-blue:   #3b82f6;
+    --env-color-purple: #a855f7;
+    --env-color-pink:   #ec4899;
+}
+
+[data-theme='dark'] {
+    --env-color-slate:  #a1a9b8;
+    --env-color-red:    #f87171;
+    --env-color-amber:  #fbbf24;
+    --env-color-green:  #34d399;
+    --env-color-teal:   #2dd4bf;
+    --env-color-blue:   #60a5fa;
+    --env-color-purple: #c084fc;
+    --env-color-pink:   #f472b6;
+}