be sailing under false colors

To be operating under the guise of false pretenses, so as to deceive someone or to hide one's true nature or intentions. (An allusion to the identifying flags of a ship.) Tim thought he could just put on fancy clothes and rub elbows with the upper crust that Janet's family socialized with, but everyone at the party knew he was sailing under false colors. Lest you think I'm sailing under false colors, let me say straight away that I'm being paid to give a review of this product today. Please, Senator, we know you're sailing under false colors. You have no intention of actually helping our small town, you just want our votes.

be sailing under false colours

BRITISH

If someone or something is sailing under false colours, they are deliberately deceiving people. Note: A ship's colours are its national flag. This report sails under false colours. It claims to be a fair and rigorous examination of the issue, but it is no such thing. Note: When pirate ships spotted a treasure ship, they often took down their own flag and raised the flag of a friendly nation, in order to get close enough to the ship to attack it.