show one's true colors, to

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

show one's true colors, to

To reveal oneself frankly; to admit one’s genuine opinions or character. This term, the converse of sail under false colors, similarly alludes to the practice of deceiving the enemy by flying a friendly flag. However, it also was applied to cosmetics used to conceal or enhance a complexion. Thus Thomas Dekker wrote (A Description of a Lady by Her Lover, ca. 1632), “The reason why fond women love to buy Adulterate complexion: here ’tis read,—False colours last after the true be dead.” The metaphor was used for one’s genuine character by numerous writers, Dickens among them: “He didn’t venture to come out in his true colours” (The Old Curiosity Shop, 1840).
See also: show, true