I would think that peroxide is your answer, but you absolutely will damage the hair doing this. Also, you need to protect the rest of your horse very well. You don't want that peroxide getting around the anus especially. Also, I would coat the underside of his tail with something. Maybe vaseline? I don't know. Ask a hairdresser if that would work.
Seems like a lot of work and then it's going to grow out anyway. Personally, I wouldn't do it.
Edited to add: I really like the extensions idea from usandpets. Much safer and, more versatile too. You can change the colours according to your mood!
Last edited by NorthernMama; 07-29-2012 at 10:24 AM.