diff --git a/frontend/src/features/projects/ProjectSettingsPanel.test.tsx b/frontend/src/features/projects/ProjectSettingsPanel.test.tsx
index 1832968f..d8dbbee4 100644
--- a/frontend/src/features/projects/ProjectSettingsPanel.test.tsx
+++ b/frontend/src/features/projects/ProjectSettingsPanel.test.tsx
@@ -43,6 +43,108 @@ describe("ProjectSettingsPanel", () => {
     window.removeEventListener(PROJECT_THEME_CHANGED_EVENT, changed);
   });
 
+  it("applies the colour immediately while persisting it for the opaque project id", async () => {
+    const gateways = createMockGateways();
+    const project = await gateways.project.createProject("PipoufWebSite", "/website-pipouf");
+    const pendingSave = deferred<{ themeColor: string | null }>();
+    const update = vi
+      .spyOn(gateways.project, "updateProjectSettings")
+      .mockReturnValue(pendingSave.promise);
+    const changed = vi.fn();
+    window.addEventListener(PROJECT_THEME_CHANGED_EVENT, changed);
+    render(
+      <DIProvider gateways={gateways}>
+        <ProjectSettingsPanel projectId={project.id} />
+      </DIProvider>,
+    );
+
+    const picker = (await screen.findByLabelText("Couleur du projet")) as HTMLInputElement;
+    await waitFor(() => expect(picker.disabled).toBe(false));
+    fireEvent.change(picker, { target: { value: "#613583" } });
+
+    expect(update).toHaveBeenCalledWith(project.id, { themeColor: "#613583" });
+    expect(update).not.toHaveBeenCalledWith("PipoufWebSite", expect.anything());
+    expect(changed).toHaveBeenCalledWith(
+      expect.objectContaining({
+        detail: { projectId: project.id, themeColor: "#613583" },
+      }),
+    );
+    expect(picker.value).toBe("#613583");
+
+    pendingSave.resolve({ themeColor: "#613583" });
+    await waitFor(() => expect(picker.disabled).toBe(false));
+    window.removeEventListener(PROJECT_THEME_CHANGED_EVENT, changed);
+  });
+
+  it("reloads the persisted colour after the settings surface is reopened", async () => {
+    const gateways = createMockGateways();
+    const project = await gateways.project.createProject("PipoufWebSite", "/website-pipouf");
+    const first = render(
+      <DIProvider gateways={gateways}>
+        <ProjectSettingsPanel projectId={project.id} />
+      </DIProvider>,
+    );
+
+    const picker = (await screen.findByLabelText("Couleur du projet")) as HTMLInputElement;
+    await waitFor(() => expect(picker.disabled).toBe(false));
+    fireEvent.change(picker, { target: { value: "#613583" } });
+    await waitFor(() => expect(picker.disabled).toBe(false));
+    first.unmount();
+
+    render(
+      <DIProvider gateways={gateways}>
+        <ProjectSettingsPanel projectId={project.id} />
+      </DIProvider>,
+    );
+    await waitFor(() =>
+      expect((screen.getByLabelText("Couleur du projet") as HTMLInputElement).value).toBe(
+        "#613583",
+      ),
+    );
+  });
+
+  it("rolls back a failed optimistic colour and retries the same payload", async () => {
+    const gateways = createMockGateways();
+    const project = await gateways.project.createProject("PipoufWebSite", "/website-pipouf");
+    await gateways.project.updateProjectSettings(project.id, { themeColor: "#4C8DFF" });
+    const update = vi
+      .spyOn(gateways.project, "updateProjectSettings")
+      .mockRejectedValueOnce(new Error("temporary"))
+      .mockResolvedValue({ themeColor: "#613583" });
+    const changed = vi.fn();
+    window.addEventListener(PROJECT_THEME_CHANGED_EVENT, changed);
+    render(
+      <DIProvider gateways={gateways}>
+        <ProjectSettingsPanel projectId={project.id} />
+      </DIProvider>,
+    );
+
+    const picker = (await screen.findByLabelText("Couleur du projet")) as HTMLInputElement;
+    await waitFor(() => expect(picker.disabled).toBe(false));
+    fireEvent.change(picker, { target: { value: "#613583" } });
+
+    const alert = await screen.findByRole("alert");
+    expect(alert.textContent).toBe("Impossible d’appliquer la couleur. Réessayer");
+    expect(picker.value).toBe("#4c8dff");
+    expect(changed).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        detail: { projectId: project.id, themeColor: "#4C8DFF" },
+      }),
+    );
+
+    fireEvent.click(screen.getByRole("button", { name: "Réessayer" }));
+    await waitFor(() => expect(update).toHaveBeenCalledTimes(2));
+    expect(update).toHaveBeenLastCalledWith(project.id, { themeColor: "#613583" });
+    await waitFor(() => expect(screen.queryByRole("alert")).toBeNull());
+    expect(picker.value).toBe("#613583");
+    expect(changed).toHaveBeenLastCalledWith(
+      expect.objectContaining({
+        detail: { projectId: project.id, themeColor: "#613583" },
+      }),
+    );
+    window.removeEventListener(PROJECT_THEME_CHANGED_EVENT, changed);
+  });
+
   it("clears a transient load error and retries the current project", async () => {
     const gateways = createMockGateways();
     const project = await gateways.project.createProject("Alpha", "/alpha");
diff --git a/frontend/src/features/projects/ProjectSettingsPanel.tsx b/frontend/src/features/projects/ProjectSettingsPanel.tsx
index 4db4a571..b389c4bc 100644
--- a/frontend/src/features/projects/ProjectSettingsPanel.tsx
+++ b/frontend/src/features/projects/ProjectSettingsPanel.tsx
@@ -17,6 +17,9 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
   const [themeColor, setThemeColor] = useState<string | null>(null);
   const [busy, setBusy] = useState(true);
   const [error, setError] = useState<string | null>(null);
+  const [failedSave, setFailedSave] = useState<{
+    themeColor: string | null;
+  } | null>(null);
   const loadSequence = useRef(0);
   const saveSequence = useRef(0);
   const currentProjectId = useRef(projectId);
@@ -29,6 +32,7 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
     const themeRevision = themeRevisionRef.current[loadedProjectId] ?? 0;
     setBusy(true);
     setError(null);
+    setFailedSave(null);
     setThemeColor(null);
     try {
       const settings = await project.readProjectSettings(loadedProjectId);
@@ -37,7 +41,8 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
         currentProjectId.current !== loadedProjectId ||
         (themeRevisionRef.current[loadedProjectId] ?? 0) !== themeRevision
       ) return;
-      setThemeColor(validThemeColor(settings.themeColor));
+      const loaded = validThemeColor(settings.themeColor);
+      setThemeColor(loaded);
     } catch {
       if (
         loadSequence.current !== sequence ||
@@ -75,11 +80,19 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
   async function save(next: string | null) {
     const savedProjectId = projectId;
     const sequence = ++saveSequence.current;
+    const previous = themeColor;
+    const requested = validThemeColor(next);
     setBusy(true);
     setError(null);
+    setFailedSave(null);
+    setThemeColor(requested);
+    publishProjectThemeChanged({
+      projectId: savedProjectId,
+      themeColor: requested,
+    });
     try {
       const saved = await project.updateProjectSettings(savedProjectId, {
-        themeColor: next,
+        themeColor: requested,
       });
       const normalized = validThemeColor(saved.themeColor);
       if (
@@ -93,11 +106,17 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
         themeColor: normalized,
       });
     } catch {
+      publishProjectThemeChanged({
+        projectId: savedProjectId,
+        themeColor: previous,
+      });
       if (
         currentProjectId.current === savedProjectId &&
         saveSequence.current === sequence
       ) {
-        setError("Impossible d’enregistrer la couleur du projet.");
+        setThemeColor(previous);
+        setError("Impossible d’appliquer la couleur.");
+        setFailedSave({ themeColor: requested });
       }
     } finally {
       if (
@@ -131,8 +150,14 @@ export function ProjectSettingsPanel({ projectId }: { projectId: string }) {
         {busy && <Spinner size={14} />}
         {error && (
           <div role="alert" className="flex flex-wrap items-center gap-2 text-sm text-danger">
-            <span>{error}</span>
-            <Button size="sm" variant="secondary" onClick={() => void load()}>
+            <span>{error} </span>
+            <Button
+              size="sm"
+              variant="secondary"
+              onClick={() =>
+                failedSave ? void save(failedSave.themeColor) : void load()
+              }
+            >
               Réessayer
             </Button>
           </div>