>medieval

When, because that's about a thousand years.

>to wear a leather vest+skirt combo over a full chainmail suit?

I don't think I've ever heard of such a thing.

>Or would it be more common to just wear some clothes over the mail if they wanted to show the colors of their master?

You might be thinking of a surcoat, jupon, or tabard, depending upon the era. These could be simple colours or embroidered, and while a squire would want to show his allegiance, for most of that era a man-at-arms wouldn't get the same sort of consideration, and many of them would be lucky to get a 'uniform'.