Theres a lot posted so if I say something someone else has already said sorry.
My horses tails are black. But I treat them like their white, because my little boy is a rubber.
Every week I wash their tails[shampoo-condition-small amount of baby oil] and French braid them, that keeps the top of the tail from breaking. If your horse is in a stall you can get a tail bag, and tie that into the tail. But I don't recommend that if your horse is a pasture, because they tend to fall out or get ripped out. So I'd use old pantie hose, or an old tube sock. [Depending on the length of your horses tail.
Also when your washing, if it's cold to use the wash racks or you keep your horses at your house, you can use Cowboy magics shower in a bottle and a small amount of your horses conditioner. =] That way your horses toshey doesn't get all wet and cold =]