Spikecast
Super Freak
I'm not painter myself, but my Wolvie had a tiny paint chip in the blue shoulder area. So I just bought some blue acrylic paint to touch it up. Luckily mine had a damaged hand (one bent claw), so I was able to recieve a replacement. Although I smashed the original, I kept a larger peice so I could make sure the blue paint matched first beforehand.
You said yours was just a chip on the black part of the mask, that shouldn't be too tough. Just any black acrylic paint should do. Just make sure to use a tiny, tiny brush.
Thanks for the advice man. I'll get myself some paint