Well, if I were doing this I would put down flat black spray paint, two layers making sure to cover all parts, and then I'd take a dark blue a very lightly go over different random spots, just enough paint to be visible against the black, then I'd put a good amount of salt, I would use a salt that has bigger grains, all over the blaster, like sprinkled all over it. Then, take a brighter green, not too bright but brighter than the blue, and put that on two layers across the whole thing, after 48 hours take a cheap dollar store toothbrush and brush 90% of the salt off, after another 24 hours I would give it two thin coats of clear coat.
Any spray paint should work, I don't recommend using any type of Rust-Oleum paint or clear coat, and I would use all flat colors. Remember to let the paint dry for about 5 - 10 minutes between coats, the can will say 3 minutes, but it really needs more like 5 to 10.
Then I would get rid of the front barrel, if I were in a Zombie Apocalypse I would want something small and agile to dodge the zombies.
I would also add a flip mag or mag connecter that allows two mags to be hooked together.
I would probably tape off the handle after the black paint and keep it taped up until you clear-coat the blaster. I would probably take some putty and fill in the bottom of the handle to be more comfortable for my larger hands but I wouldn't recommend you to do that.
I will also say that I haven't used the salt trick before but I've seen it used and can look really cool. I am planning on using it for a build I'm working on now.
I hope this helps and I'll let you know if I have anymore ideas.
--Captain Shotgun