Sand the grips so that the surface is rough, prime it with a coat or two, wait to dry. Krylon has some great camo ultra flat paints that i would recommend.
I have painted my Tokyo Marui ak-47 olive drab colour with krylon paints. It looks great and is just wearing off a bit after a year and a half.
After painting the surface will feel rough and sandy because of the paint. I used some sort of silicone based car paint cleanser thing and it worked amazingly. After that and some wearing in from holding it so long it looks and feels like it came with the gun.
EDIT: Heres a picture of the finished paintjob