From Longman Dictionary of Contemporary Englishyour true coloursyour true coloursif you show your true colours, you do something which shows what your real attitudes and qualities are, especially when they are badshow/reveal your true colours He was forced to reveal his true colours when asked how he would vote. → true
Examples from the Corpus
show/reveal your true colours• Archer is proof of the adage that when people write fiction, they show their true colours.• Well, it hadn't taken him long to show his true colours!• I didn't show my true colours until they made me chairman. 