I used to have a snow white mare and I SWEAR by a product called Orvus. Years ago I was best friends with a dairy farmer's son who showed cows nationally in Canada and in Madison, Wisconsin. Cows LOVE to lay in their pee and everything, so one day on a whim I asked him what they used to clean them.
Orvus is GREAT and it works. I use about a tablespoon (I have an old measuring spoon in the bucket) for each of my horses socks and his blaze. All you do is rub it on with a brush, rinse it off. It'll be so white it'll blind you!
I bought the bucket, which I believe to be 8 pounds, for around $30.. four years ago.. I think I still have almost 3/4 of it left.
I know one bucket used to do about a eighteen Holstein dairy cows haha.
As for Cowboy Magic, I tried every product to whiten in their line and was thoroughly unimpressed with them all. Orvus gets my horse so shiny, I contemplate washing myself with it.