I have a leopard spot POA and a bald faced paint who like to get pretty grubby and don't like having their faces hosed so I get either Quiksilver (horse shampoo for white areas) or a bottle of Miracle Groom cleaner and a washcloth, get the washcloth very wet and use that to get their faces wet, then add some of the cleaner and scrub with that, then get the washcloth really wet and keep squeezing it on their face until they are rinsed. This works, it just takes time. If your horse tolerates the hose to their face, then just use that with the Quiksilver which works wonders!
In the summer for shows I clip the white on my horse's face with a 10 blade. It makes the white look much neater and it makes it much easier to clean. I als like Cowboy Magic Green Spot Remover.