To reveal one’s real character or intentions, especially when these are disreputable or dishonorable:

She was only too anxious to get out of the room now that her employer had shown his true colors.

In some ways it’s a real blessing that Bill showed his true colors relatively early in our relationship.

There is effective suspense, sure, but the villains are clearly defined, and any ambiguous characters immediately show their true colors.

Polly starts out as an innocent young woman swept away by the romance of a wedding, but soon shows her true colours by taking over her husband’s old firm of gangsters as soon as he is out of the way.