I use QuicSilver on Lacey and oh man. That stuff is my BFF. Haha On her body it only needed to sit for a few minutes to get her really white but her tail needed a full 20 minutes (it only said 15 on the bottle) to get really white. It was very worth the wait though because her tail was BEAUTIFUL when I rinsed the stuff out.
I cleaned and oiled tack while I was waiting which worked out really well because I could keep an eye on Lacey (I had her tied up) but also be doing something worthwhile.
Pictures! (I know it's the opposite problem you're having but it might give you hope) =)
Before:
After:
Good luck! =)