diff --git a/apps/theming/lib/Command/UpdateConfig.php b/apps/theming/lib/Command/UpdateConfig.php
index 849744077ef42..e2592c4dda414 100644
--- a/apps/theming/lib/Command/UpdateConfig.php
+++ b/apps/theming/lib/Command/UpdateConfig.php
@@ -128,6 +128,11 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 			$key = $key . 'Mime';
 		}
 
+		if ($key === 'color' && !preg_match('/^\#([0-9a-f]{3}|[0-9a-f]{6})$/i', $value)) {
+			$output->writeln('<error>The given color is invalid: ' . $value . '</error>');
+			return 1;
+		}
+
 		$this->themingDefaults->set($key, $value);
 		$output->writeln('<info>Updated ' . $key . ' to ' . $value . '</info>');
 
diff --git a/apps/theming/lib/ThemingDefaults.php b/apps/theming/lib/ThemingDefaults.php
index 19c20be28e1fb..59f32d0a8168f 100644
--- a/apps/theming/lib/ThemingDefaults.php
+++ b/apps/theming/lib/ThemingDefaults.php
@@ -213,7 +213,11 @@ public function getShortFooter() {
 	 * @return string
 	 */
 	public function getColorPrimary() {
-		return $this->config->getAppValue('theming', 'color', $this->color);
+		$color = $this->config->getAppValue('theming', 'color', $this->color);
+		if (!preg_match('/^\#([0-9a-f]{3}|[0-9a-f]{6})$/i', $color)) {
+			$color = '#0082c9';
+		}
+		return $color;
 	}
 
 	/**