If you are going for solid colors, I think that FDE, foliage green or OD green look good. You could even do that in some sort of combo.
Honestly I don't like painting guns, but if you go that route and do a complete camo job, a lot of paintball fans LOVE predator camo. My uncle swears by predator when bow hunting and paintball - he wears a light-print predator t-shirt over a darker pattern predator long-sleeve shirt, it off-sets your torso.... It makes you hard to see even semi-exposed because human eyes are looking for something blocking light, and if you see lighter colors with the foliage it doesn't look like anything is blocking the light....
Also, considering camos usually have seasonal patterns, if you go with solid colors instead of camo paint pick your colors based on the season/environment you'll be in most.
EDIT: There was a guy on here a while back, can't remember when or his name, but he had a thread showing his paint jobs on different AR15 rifles. I think he was doing it in camo patterns and solid colors, but he was picking the paint both for how well it concealed, as well as how well it worked in night vision. His pics in the day vs. nightvision were unbelievable! With NV those guns were straight invisible, even propped against a white fence!