Use dog whitening shampoo, I have heard of ketchup but have never personally used it before.
Purple shampoo (like white and bright), then the magic is orvus. It's a livestock bleach that makes the worst stained tails white again. If you use it (or any other livestock bleach) be sure to not use it on their skin, condition their tails and wash your hands afterwards because it is very strong.
Purple then I would use the cowboy magic conditioner
For the grey horse that used to have I used the quick silver and the vetrolin white and bright worked the best for me!
What I did was shampoo with the cowboy magic rose water shampoo, then I did the quick silver or the white and white and let it set for no more than 10 min or it will turn the tail
Ok so get grey horse shampoo and a bottle of vinegar. Put the vinigar in first then the purple shampoo. Scrub and let it sit for 15 mon