GOOP! Works wonders, and you can get a big tub of it for less than $2 at Wal-Mart. Smells great, too! Rub it on DRY hair, let sit for 15+ minutes, than add shampoo and wash and condition. Love it! Even my no frills farrier had to try it after I told her about it, and she doesn't care about things like white tails, lol.
Also, there is no risk for you to get distracted, and turn your horse blue.
"On the back of a horse I felt whole, complete, connected to that vital place in the center of me...and the chaos within me found balance."
Funny enough, the same thing that works getting yellow pit stains out of white shirts helps with white tails. Mix some Dawn dish soap with a quarter cup or so of hydrogen peroxide and a tablespoon or so of baking soda. Make a goop and lather the tail with it, then rinse!
I think you can also create a mixture of Dawn and Oxyclean, it's about the same thing. The liquid peroxide makes a better paste though.
It works miracles of white shirts too! Saved me a few nice dress shirts, couldn't believe it.
I had a gray mare come here to foal and her tail was stained reddish brown. She was a little underweight and rough-coated so i added a cup of flax a day to her normal "broodmare" ration--her tail, as it grew out, was BRIGHT WHITE and stayed that way up top where the new growth was, in spite of her continuing to be otherwise quite the little piggy, wallowing in dirt regularly.
I have two pintos, and when I keep them on a decent amount of flaxseed they shed stains really easily and stay surprisingly clean.
How much Flax did you feed? Do you buy it ready to serve or grind it yourself?
The mare's tail is still yellow, so I went to Sally's Beauty Supply and explained to them what I wanted to do. They gave me "White-Out Kit, Bleach and Toner" made by N Rage. They told me some of the dog show people use this to get stains off their white dogs. Sunday is supposed to be warm here so I'm going to try it. I'll let you know if it fixes her yellow tail. (I will also add that I've only had this mare a short time so I haven't been able to make a difference in her nutrition yet. I'm just looking for something that can fix the immediate yellow tail problem now and then I'll work on getting it to grow out nice and healthy.) Thanks!
Update: the bleach and toner worked! Her tail is very white with only a tiny bit of yellow at the ends. It was also very easy to do. I would recommend getting a friend to help you as it is best as a two person job.