show one's true colors

The term is of nautical origin, derived from the requirement that every ship fly an ensign, or colours, to indicate its national allegiance while at sea. It was a standard deception among pirates to sail under false colours, hoisting a friendly flag to approach a merchant vessel without causing alarm. The tactic relied on the target remaining unaware of the pirate’s intent until the distance between the two ships had closed sufficiently to make escape impossible.

Only when the pirate ship reached close quarters would it unfurl its true colours. The sudden replacement of a friendly flag with the pirate’s own ensign served as the final signal of hostile intent. The phrase entered the wider language as a metaphor for the revelation of a person's actual character or motives after a period of calculated misdirection.