I LOVE the Quic shampoos. I use Quic silver on my paint pony and it gets him really clean. I also have bought Shapely's Equitione shampoo. I have not gotten to use it yet, but it looks like it will work.
You probably only need to get one shampoo. I would get one made for your horse's main color(bay, did you say?). If you really want to make any white areas whiter, use baking soda or highlite them with grooming chalk.