show (one's) (true) colors
To reveal what one really believes, thinks, or wants; to act in accordance with one's real personality, temperament, 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 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 show their colors.
show one's (true) colors
Fig. to show what one is really like or what one is really thinking. Whose side are you on, John? Come on. Show your colors. It's hard to tell what Mary is thinking. She never shows her true colors.