show one's true colors

Idioms

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]

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023