To see someone as they really are

I thought he was a friend, then I saw his true colors: my job was at stake and he didn't do a damn thing to help me out

Pensavo fosse un amico, ma poi ho capito chi era veramente: il mio lavoro era a rischio e lui non ha fatto assolutamente niente per aiutarmi

Now that I found out what you are up to, I see your true colors!

Adesso che ho scoperto le tue vere intenzioni, finalmente vedo la tua vera faccia!

I thought he was Mr. Right, but after the terrible scene he made last night over nothing, I think I saw his true colors and I don't like it at all

Pensavo fosse il principe azzurro, ma dopo l'assurda scenata che mi ha fatto ieri sera senza motivo, credo di aver visto la sua vera faccia e non mi piace per niente

One of the uses of the word colors is flag, pennant. Early warships often carried flags from many nations on board in order to elude or deceive the enemy. The rules of civilized warfare called for all ships to hoist their true national ensigns before firing a shot. Someone who finally "shows his true colors" is acting like a warship which hails another ship flying one flag, but then hoisted their own when they got in firing range