SAIL UNDER FALSE COLORS

v. phr. 1. To sail a ship, often pirate, under the flag of another country. The pirate ship flew the American flag until it got near, then raised the black flag. 1. To pretend to be what you are not; masquerade. The garage hired Jones as a mechanic, but fired him when they found he was sailing under false colors. They found out that Smith was an escaped convict who had been sailing under false colors as a lawyer.