There was a style that became popular in 16th century Germany referred to as "black and white" armor. Decorative patterns were created by selectively polishing certain areas. Here's a good example. As you might expect, the higher one's status, the fancier of a pattern one could afford. This, by contrast, is a relatively "budget" example.

Oh yes. When I first read your question I thought of enameled armor but some poking around turned up this wonderful article from The Met. Depending on the fantasy story, I doubt armor was quite as colorful (at least anything that wasn't gold, silver, or bronze, but as the article details there were many techniques to add color) but it could get extremely elaborate and could have color.

Here's a helmet from about 1500 that has a monster face painted on the visor. It's also unusual as it's an example of relatively low-quality 'munition' armour, as opposed to the higher quality examples that usually survive. I also came across this album on imgur that contains a couple of examples of painted sallets (along with some modern reproductions).

A slightly-off topic, but cool example that you can find mentioned is in David Parrott's The Business of War (at least where I encountered it first) are the Landsknecht of the early modern Europe period in Germany. They were known to be particularly colorful in terms of dress both in and out of battle. Many old representations of them show them in very ostentatious, bright and what we today might consider to be gaudy dress. Their dress was very distinctive for their time, with feathers, hats, etc., that would tell anyone and everyone of their profession at a glance. Not quite the question you asked, but I figured I'd turn you onto other kinds of military dress.

In The Armourer and His Craft, ffoulke describes armour covered with velvet - not like a slipcover, but a permanent surface. Boiled linseed oil, like oil paint, could be tinted before being used as water-proofing.