Purple shampoo or baby powder should work
Fairy liquid gets my grey nice and clean also gallop goop is best
Tomato Ketchup. Dampen his feathers and then rub in the ketchup. Leave for 20 mins and then wash out with a mixture of vanish/washing up liquid, or a whitening shampoo. We do it with our flaxen pony on her tail. It comes up amazing.
I used washing up liquid on my white Friesian X. Worked like a charm x
Phoebe is right! Purple shampoo is awesome!
Have you tried purple shampoo? It's brilliant!