True Color

[morning]

Hi, i often see sentences like, Children always show their true color. I take it as a positive comment because it seems to mean children never lie about their feeling or so.

However, can I use 'true color' as follows:

After John won the fashion competition, his best friend, who has been jealous of John's success, finally showed his true color and wrote an disapproving review.

In this example, 'true color' becomes the 'real face' of the best friend. If it is not correct to use 'true color' in this example, what words/phrase should I use?

thanks!

[morning]

Any advise? Or, should I put this under Idiom section? Thanks!