Showing one's true colors applies to a situation where a person did something that you perceive as negative.
For instance: I thought Jake was a nice guy, but at the club, he showed his true colors.
Is there an idiom which would convey the idea that what a person did led you to perceive them positively thereafter? Consider: Tim was initially shy, but once we worked on that he proved to be an amazing guy.
If not, is there a better way to say this?