show someone in their true colors

US /ʃoʊ ˈsʌm.wʌn ɪn ðer truː ˈkʌl.ərz/
UK /ʃoʊ ˈsʌm.wʌn ɪn ðer truː ˈkʌl.ərz/
"show someone in their true colors" picture
1.

montrer quelqu'un sous son vrai jour, révéler la vraie nature de quelqu'un

to reveal someone's true character, especially if it is negative or unpleasant

:
The crisis really showed him in his true colors.
La crise l'a vraiment montré sous son vrai jour.
His actions eventually showed him in his true colors to everyone.
Ses actions l'ont finalement montré sous son vrai jour à tout le monde.