If you want to keep the stock grips, I have a suggestion for you. You can dye them, and the color will not bleed. This is a trick used by people who do HK G36 conversions.
Get some RIT fabric dye. It must be the liquid, not the powder. You will need to mix a ratio of 3 parts black to one part green. There are many colors of green from RIT. The purpose of the green is to keep the finished product from looking too purple. Anyway, I get RIT forrest green, Kelly green, or whatever, just as long as it's not a light green. You can buy one container of black and one of green, and mix the proper ratio, as it's not cheap, but be sure to do it outside as there will certainly be splash issues.
Bring the stuff to a boil with the most minimal amount of water possible in a container that will allow you to completely sumberge your grips. Put the stuff in a small pot or whatever, and heat it. Add a little salt to facilitate boiling at a lower temp. Once it comes to a boil, take it off the heat and drop your grips in. let it set for 6 hours. Take out the grips, wash in water using a toothbrush to gently scrub out any surface residue, and you will have black grips. If you do this, let us know how it turns out. It will be much more durable and permament than paint.