Cowboy Magic Yellow Out shampoo or their green spot remover
I use a blue and white soap (bar soap) and it's perfect, I wash legs and tail with it, they are super white and it prevents bacterial problems!
Hope it Helps
K9 sterling silver shampoo works wonders on my grey horses, and green spot remover (k9, leovet and horseware make good ones)
First clip the white so there is less hair for the dirt to stick to. Then use green spot remover to remove the dirt and stains then rub in some baby powder with a towel to make them look super white and clean.