There are some color enhancing supplements that work pretty well. Black as Knight is one used on dark horses. It's made by Cheval International, and they make one for golden horses too, but the name escapes me. You could also try a general coat or hoof supplement, like Nu Image (just an example that I have experience with--there are a lot of them on the market). You could also use rice bran, rice bran oil, wheat germ oil, flax seed, etc.
There are a lot of whitening shampoos on the market, as well. If you go to any major equine retailer on the internet you can find a bunch. Search Results - Navigator Search Results - Navigator
Valley Vet is my favorite site, but there are many others. Dover Saddlery, Smith Brothers, and State Line Tack are three off the top of my head. And don't overlook the importance of grooming in getting that shine!