under false colors
Using or 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, and so usually used in the phrase "sail under false colors.") Primarily heard in US. 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. I don't want to be accused of flying under false colors, so let me say straight away that I'm being paid to give a review of this product today.