diff --git a/CHANGELOG.md b/CHANGELOG.md
index fb8b8fdee..21ae948d0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,9 @@ You should also include the user name that made the change.
 - update dependencies @syuilo
 - enhance: display URL of QR code for TOTP registration @syuilo
 - make CAPTCHA required for signin to improve security @syuilo
+- The theme color is now better validated. @Johann150
+  Your own theme color may be unset if it was in an invalid format.
+  Admins should check their instance settings if in doubt.
 - Perform port diagnosis at startup only when Listen fails @mei23
 
 ### Bugfixes
diff --git a/packages/backend/migration/1652859567549-uniform-themecolor.js b/packages/backend/migration/1652859567549-uniform-themecolor.js
new file mode 100644
index 000000000..bc47143e5
--- /dev/null
+++ b/packages/backend/migration/1652859567549-uniform-themecolor.js
@@ -0,0 +1,38 @@
+import tinycolor from 'tinycolor2';
+
+export class uniformThemecolor1652859567549 {
+	name = 'uniformThemecolor1652859567549'
+
+	async up(queryRunner) {
+		const formatColor = (color) => {
+			let tc = new tinycolor(color);
+			if (color.isValid()) {
+				return color.toHexString();
+			} else {
+				return null;
+			}
+		};
+
+		await Promise.all(queryRunner.query('SELECT "id", "themeColor" FROM "instance" WHERE "themeColor" IS NOT NULL')
+		.then(instances => instances.map(instance => {
+			// update theme color to uniform format, e.g. #00ff00
+			// invalid theme colors get set to null
+			instance.color = formatColor(instance.color);
+
+			return queryRunner.query('UPDATE "instance" SET "themeColor" = :themeColor WHERE "id" = :id', instance);
+		})));
+
+		// also fix own theme color
+		await queryRunner.query('SELECT "themeColor" FROM "meta" WHERE "themeColor" IS NOT NULL LIMIT 1')
+		.then(metas => {
+			if (metas.length > 0) {
+				return queryRunner.query('UPDATE "meta" SET "themeColor" = :color', { color: formatColor(metas[0].color) });
+			}
+		});
+	}
+
+	async down(queryRunner) {
+		// The original representation is not stored, so migrating back is not possible.
+		// The new format also works in older versions so this is not a problem.
+	}
+}
diff --git a/packages/backend/src/server/api/endpoints/admin/update-meta.ts b/packages/backend/src/server/api/endpoints/admin/update-meta.ts
index b23ee9e3d..09e43301b 100644
--- a/packages/backend/src/server/api/endpoints/admin/update-meta.ts
+++ b/packages/backend/src/server/api/endpoints/admin/update-meta.ts
@@ -27,7 +27,7 @@ export const paramDef = {
 		blockedHosts: { type: 'array', nullable: true, items: {
 			type: 'string',
 		} },
-		themeColor: { type: 'string', nullable: true },
+		themeColor: { type: 'string', nullable: true, pattern: '^#[0-9a-fA-F]{6}$' },
 		mascotImageUrl: { type: 'string', nullable: true },
 		bannerUrl: { type: 'string', nullable: true },
 		errorImageUrl: { type: 'string', nullable: true },
diff --git a/packages/backend/src/services/fetch-instance-metadata.ts b/packages/backend/src/services/fetch-instance-metadata.ts
index d5294c5fe..029c388dc 100644
--- a/packages/backend/src/services/fetch-instance-metadata.ts
+++ b/packages/backend/src/services/fetch-instance-metadata.ts
@@ -1,5 +1,6 @@
 import { DOMWindow, JSDOM } from 'jsdom';
 import fetch from 'node-fetch';
+import tinycolor from 'tinycolor2';
 import { getJson, getHtml, getAgentByUrl } from '@/misc/fetch.js';
 import { Instance } from '@/models/entities/instance.js';
 import { Instances } from '@/models/index.js';
@@ -208,16 +209,11 @@ async function fetchIconUrl(instance: Instance, doc: DOMWindow['document'] | nul
 }
 
 async function getThemeColor(doc: DOMWindow['document'] | null, manifest: Record<string, any> | null): Promise<string | null> {
-	if (doc) {
-		const themeColor = doc.querySelector('meta[name="theme-color"]')?.getAttribute('content');
+	const themeColor = doc?.querySelector('meta[name="theme-color"]')?.getAttribute('content') || manifest?.theme_color;
 
-		if (themeColor) {
-			return themeColor;
-		}
-	}
-
-	if (manifest) {
-		return manifest.theme_color;
+	if (themeColor) {
+		const color = new tinycolor(themeColor);
+		if (color.isValid()) return color.toHexString();
 	}
 
 	return null;