expression, ichidan verb
reveal one's true colors; show one's true nature
Idiomatic expression literally meaning 'the gold leaf peels off', used when someone's superficial good qualities wear away and their real, often less admirable, character is exposed.
See also: メッキが剥げる (メッキがはげる)
As you spend more time together, anyone's true colors will show.
彼の親切も長くは続かず、すぐに金箔が剥げた。
His kindness didn't last long; his true nature soon came out.