Try Jim's Gun Supply for all your colorful gun needs! It starts getting weird about halfway down the page. Sadly, they don't actually sell these guns (these are photos of some of their custom work), but they will colorize your weapon however you want. They have a whole section of just pink guns and accessories!
It seems like I recall there being a law now that toy guns, water pistols, etc. cannot be colored like real guns (or maybe that was a reaction from the toy weapons industry). I don't suppose there's a law that says real weapons can't look like toys, is there?