To reveal one's true intentions and/or personality

During her trial period, Lucille was extremely nice and helpful. Once she was hired, she showed her true colors: she's a social climber who, for her career, is ready to walk all over anyone and everyone

Durante il periodo di prova, Lucilla era gentilissima e disponibile; una volta assunta definitivamente ha mostrato la sua vera faccia: è un'arrampicatrice sociale che non guarda in faccia a nessuno pur di far carriera

In an off-the-record interview, the famous crooner, showing his true colors, finally opened up and said that he was at odds with his label because he considers himself as a rock singer and hates the musical choices the agency is making for him

In un'intervista non ufficiale, il famoso cantante melodico si è rivelato e ha dichiarato di essere ai ferri corti con la propria casa discografica perché, in realtà, lui si sente un cantante rock e non è assolutamente d'accordo con le scelte che fanno per lui

So that's what you are up to! Well, you are finally showing your true colors!

Così queste sono le tue reali, opportunistiche intenzioni! Finalmente mostri la tua vera faccia!

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