[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./