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.