i love orvus, technically its a cattle shampoo but works amazing on horses as long as you rinse it all out afterwards!
I've had good luck with the Vetrolin whitening shampoo as well
For my greys what I do is I will wash them with the vetrolin White N Brite shampoo and let it sit for a minute or two. Then I'll rinse and do the gallop shampoo for greys. Again let that sit.... (they're both purple shampoos) rinse that out... and what I try to do is clip them as often as I can because that the only way to 100% get off stains. And green spot remover is great!! Basically you will spend a lot of time grooming, (curry and a hard brush will be your best friend) and you'll also spend a lot of time batheing, clipping etc. If you want them to stay show ready white most of the time. Hope this helps good luck!!
Purple shampoo and green spot remover
I think it's from a couple different brands but purple shampoo. It works great on stains.