Are there similar figurative expressions in English to Japanese proverb, “メッキが剥げる – the gilding peels off” to describe to show one’s true color?

It's common to call this showing one's true colors similar to the way you described it in your question. Wiktionary:

Verb
show one's true colors

1. (idiomatic) To reveal how one really is, as opposed to how one has been portrayed or after having been deceptively and deliberately misleading.

The usual form is colors rather than color, because as Wiktionary goes on to say, the expression is of nautical origin:

The word 'colors' (or 'colours') refers to the flag or ensign which every ship is obliged to fly at sea. It was once a common deception of pirates to 'sail under false colours' and fly a friendly flag in order to get within close range of potential targets (other ships) without exciting suspicion. Only when the pirate ship reached close quarters would it unfurl its 'true colours'.

To be completely clear, a ship's flag is always referred to as its "colors" in the plural, certainly because a flag generally has multiple colors which define it.

I'm going to suggest the idiom the mask slips. Here's an example of the usage from The Handbook of Communication and Aging Research by J Nussbaum and J Coupland

Signs of the unchanging element of identity— the inner self— can occasionally be glimpsed when the mask slips....