Is there a positive counterpart of "Showing one's true colors"?

Solution 1:

shine through is a common way of latent or hidden ability coming on display

shine through at Cambridge.org

— phrasal verb with shine UK ​ /ʃaɪn/ US ​ /ʃaɪn/ verb shone or shined ​ If a quality shines through, it is strong and easy to see, usually in a particular situation:

Take off your make-up and let your natural beauty shine through.

She is a quiet woman but her passion shines through in her music.

It took three games starting after Bledsoes injury for the first glimpses of Tom Brady's brilliant future to shine through.

Solution 2:

Consider saying that Tim revealed hidden depths.

hidden depths phrase Admirable but previously unnoticed qualities. ‘his solo spots reveal hidden depths’ - ODO

Solution 3:

I thought Jake was a nice guy, but at the club, he was a revelation.

TFD(idioms):

be a revelation
To be different than one anticipated, often in a good way.
Dana's performance in the play was a revelation—I had no idea she was such a talented actress.

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.