If you can't bath, a groomers stone (a pumice stone) works great, and if you need a little bit of extra coverup, baby powder is awesome
White and bright is a godsend but not cheap--I've found that Ivory Snow baby laundry detergent works just as great if not better, and is MUCH cheaper :)
rub it with alcohol! works like magic
You can use dry shampoo! I know pumice-stones can be really effective :)
I also use a pumice-stone on the fleshy parts of the body and dry shampoo.
When I ride a dirty grey horse, I use a pumice-stone for the worst stains and it works! You can put some cream on the stain to avoid the pumice-stone hurts the horse :)
I am sure you have heard about Magic Brush. They are rather expensive as it is just plastic brush but they are actually very effective ! I was impressed ! I think it would really work on grey horses.