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.
to reveal someone's true character, especially if it is negative or unpleasant
:
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The crisis really showed him in his true colors.
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His actions eventually showed him in his true colors to everyone.