You don't need anything except GOOP. Its a white de-greasing soap that is sold in the automotive section of wal-mart. I've checked for it at autozone, k-mart, and target, but I've only found it at wal-mart.
You rub it in/on while the horse is dry, and shampoo it off like you are giving the horse a normal bath. It will be blinding white!
I use it on my chestnut's socks. If the dirst is really stubborn, I take a rubber curry and massage the goop around before shampooing.
This is what you are looking for: Goop Hand Cleaner- America's Premium Quality Multi-Purpose Hand Cleaner Since 1949 - Original Goop
It works so well because its a de-greaser, but it will make your horses coat dull if you use it a lot on its entire body. I would just use it to problem stains, manes, and tails, or on white markings.
It is very gentle so it won't cause any irriation, but it will take out the national shine your horse has from its own oils if you over use it.