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

1.
jemanden in seinen wahren Farben zeigen, jemandes wahres Gesicht zeigen
to reveal someone's true character, especially if it is negative or unpleasant
:
•
The crisis really showed him in his true colors.
Die Krise zeigte ihn wirklich in seinen wahren Farben.
•
His actions eventually showed him in his true colors to everyone.
Seine Handlungen zeigten ihn schließlich in seinen wahren Farben allen.