It's almost impossible keeping stain off of a draft's feather for the ring, so show chalk or baby powder is used to make them bright white for a show. As far as permanently getting rid of it, I'm all ears. Nothing has worked for me.
Have you tried whitening shampoo? I know with my old grey mare, we dyed her mane and tail black to bring out her legs at shows. Maybe you could dye hers white? My horses tail is black so I don't have experience with stains :/ Posted via Mobile Device
Back home they used laundry detergent (the sensitive without perfumes type)... Since you are only washing the tail, and not rubbing it into your horses skin, it won't harm them. It's a trick the people with white horses would always do to get their tails nice and white.
I've never used it, but I guess you could research it.