SHOW ONE'S TRUE COLOURS

Reveal one's actual nature that may have been hidden. 

Example: Hollywood Reporter, review of "Tyler Perry's Temptation:

"Harley -- who owns a townhouse to die for and a private plane -- pours on the charm and ultimately seduces Judith during a business trip to New Orleans. Almost immediately afterward, he reveals his true colours as an abusive womanizer and drug and alcohol abuser."