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 colors 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
Farlex Dictionary of Idioms.

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
Farlex Partner Idioms Dictionary
See also:
  • (one's) (true) stripes
  • (one's) true color(s)
  • (one's) true colours
  • at the back of (one's) mind
  • at the back of your mind
  • be in a world of (one's) own
  • be, live, etc. in a world of your own
  • be remembered as (something)
  • be remembered as/for something
  • be written all over your face