I've had good results with recent (not ground in) stains with the Waterless shampoo in a black bottle - the name escapes me at the moment but! Spray small amount onto a small dry sponge and scrub at the stain - do NOT wet the sponge or the stain otherwise you'll just spread it! I speak from experience (see Canter/Red Rum Whitener thread below!)
A groomers stone is excellent for removing stable stains without water, even ground in ones. You just have to brush out the dust after you use it but it works really well
I would agree with Cowboy Magic .... but if recent stained then pure water nothing else.... if your horse is not shampooed and they have there natural oils it will wash off .... learned that from Portugal in training yard full of greys... and only wash in hair direction.
Cowboy Magic green spot stain remover, expensive but works. For the bottom of his tail, not dock, I use Tesco Bio detergent, probably not recommended but again, it works better than equine shampoos!
Cowboy magic or tiny spot of 'blue' shampoo on a squeezed out hot flannel, rinse flannel, squeeze out again and wipe soap out. Better on summer coats than winter and wonderful for green faces!