white and bright is great but not cheap (I've found that Ivory Snow baby laundry detergent is just as good if not better, and much cheaper). I know someone who also used to use hot water and baking soda or something along those lines to basically bleach their horses tails back to white.... something to look into.
Mane n' Tail spray & white
This stuff is pretty cheap and really works, I have a black and white tobiano paint and boy does she like mud
with ketchup (this is not a joke)
White and bright is awesome!
I find white and bright shampoo works really well.. if they doesn't work you could try livestock bleach, but it's pretty expensive.