NOOOOO! Marker bodies are generally aluminium and also made to fine tolerances. Use paint and it won't bond properly and may even stop your marker working at all. The only time I've seen it work was on the outside of something basic like a Tipman 98c, and that flaked off within a few months.
Anodising the way to go, but can be pricey - mine cost me £35 for just some of the parts - so it's by far easiest to simply buy a gun in the colours you like
unless you have a desperate desire for a special colour scheme