We used to use washing up liquid on Trigger's (12.2h Welshie) tale with a a very good rinse afterwards. It prob wasn't a v good idea but did get the stains out.
I find that cheap baby shampoo is a lot better than some of the expensive equine shampoos.
It's not a great photo, but it shows how clean I got his tail after two decent washes with baby shampoo (I did two washes as his tail was very grubby!).
I am very bad a use Ace Bleech, Have been using it for over 2 yrs and its fab, (only use on tail) Leave on for 10 mins the rinse out well and condition using hunam conditioner!