I have a "white" horse and no stranger to yellow stains and grubby tails
I do use whitening shampoo and hot water (the temperature as hot as you can handle) but don;t use it on the tail itself just the lower part where there is mainly hair
1. Soak the hair well and use ordinary soap bar - you may need to rub for awhile. Then rinse.
2.Now repeat the soap but this time after rubbing leave it in for about 5 mins. Then rinse.
3. Then take some whitening horse shampoo rub into hair well, tie up the tail and cover in plastic for about 15 mins. Then rinse and you will see the difference.
4. If the ends are still a bit yellow I suggest trimming them off to allow clean new hair to grow - shorter healthy and bright tail is better than long yellow and dull one
I would not suggest using any kind of conventional bleach - you wouldn't use it on your hair so why would you do that to your horse? It is potentially dangerous for your horses skin and breathing.