Yellow tails?

So on Sunday I'm going to a show, and this gelding's tail is a MESS. It's grey at the to fading to white. The white's all yellow/brown now.

Any tips on making it white again!?

First, just take some good shampoo and scrub, scrub, scrub. You might want to shampoo it a few times if you can't find a whitening shampoo. If you can find one, don't leave it in too long or you'll end up with a tinted tail.

I'd also suggest cleaning it on a more regular basis, and put a tail bag on it during the fall/winter months. I dealed with a palomino (with lots of chrome to boot) for two years, so I can sympathize.