I've heard that if your grey horse has dapples you can bring them out. I washed my grey with cowboy magic shampoo and that got out the toughest of stains, even when he pooed down his tail bag! Using shampoo for black horses brings out dapples in a grey :)
Quicksilver is amazing! A truly great product. , just bathe the horse super well with quicksilver, put a fly sheet on it, and wait until morning and it should be nice and clean.
Ketchup on his tail! But below the dock as the acid in the tomatoes can irritate the skin. I use human purple shampoo for grey or white haired people and also washing up liquid is brilliant also!!
Baby powder, cocunt oil and baby wipes
Also, you can use baby powder to help cover up stains! Along with baby wipes, if you scrub with those they also work!
If you don't mind to buy a rug, I will recommend to clip him. He will be so much cleaner, you don't even have to brush him... and the stains he already has will be gone bucause you will cut the stained hair of with the clipper
Orvus paste. Works like a dream
My trainer swears by ketchup.... let it sit for a while like you do the blue shampoo, and rinse out. When I clean white horses I use a mitt and lots of shampoo and scrub really hard let it sit for a while and rinse out....
Use rubbing alcohol on a rag when he's dry