sail under false colors
1. Lit. to sail with false identification. (Pirates often sailed under the national flag of the ship they planned on attacking.) The ship, sailing under false colors, suddenly started to pursue our ship. Bluebeard the pirate was known for sailing under false colors.
2. Fig. to function deceptively. You are not who you seem to be. You are sailing under false colors. Tom was sailing under false colors and finally got found out.