i found some products that are AMAZING for grey horses and or light colored horses.

Ivory dish soap. Cheap and effective.

This is common and I love it it's called Green Clean it's for grass stains but I use it for any of my grass stains plus it smells good

Tomato sauce is great for whitening light grey horses' tails. Tomatoes have bleaching properties. If you have a grey horse whose tail has a brown kinda tint to it and you'd rather it be whiter or even if you just want a white tail to glisten a bit more then I'd recommend washing it regularly with whatever you use to get rid of the topical dirt, then put the tomato sauce in from top to bottom and leave it in for 5-10 mins and then rinse out. It doesn't do anything dramatic, it just will lighten it ever so slightly (if you repeat the process a few times!) or make it really look glistening.

I have a pinto with a white tail and I use Sard stain removal spray to remove the deep stains and then I use Champion Tails premixed stain removal spray which has purple shampoo in it so leaves behind a silvery deposit :)