show one's true colors

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.
See also: color, show

show one's true colors

Reveal oneself as one really is, as in We always thought he was completely honest, but he showed his true colors when he tried to use a stolen credit card . This expression alludes to the antonym, false colors, that is, sailing under a flag other than one's own. [Late 1700s]
See also: color, show, true