the.com/true colors

means A person's real character or intentions, revealed once pretense drops away.

from Rooted in nautical signal flags — ships flew flags ('colors') to identify their nationality, and a vessel disguising itself under false flags would show its 'true colors' only when it finally revealed its real allegiance, often by raising the real flag right before attacking or declaring itself. The phrase sailed from that practical deception into everyday language for unmasking someone's genuine nature.