show (one's) (true) colours
To reveal what one really believes, thinks, or wants; to act in accordance with one's real personality, character, or disposition. Primarily heard in UK. 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 realised 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 colours.