reveal (one's) (true) colors
To reveal what one really believes, thinks, or wants; to act in accordance with one's real personality, character, or disposition. Primarily heard in US. Dave said all along that he only wanted this deal because it was in the company's best interest, but he revealed his true colors once he realized that he wouldn't get any special commission for his efforts. It's only in times of crisis that your friends will really reveal their colors.