your, his, etc. true colours

(one's) true color(s)

One's true or honest beliefs, thoughts, convictions, biases, desires, etc.; one's real personality, character, or disposition. Dave said all along that he only wanted this deal because it was in the company's best interest, but he showed his true colours once he realized that he wouldn't get any special commission for his efforts. You will really see your friends' true colors when you call upon them in a time of crisis.
See also: true

your, his, etc. true ˈcolours

(British English) (American English your, his, etc. true ˈcolors) (often disapproving) what a person is really like: Once he got into power he showed his true colours.
See also: colour, true