I've been in New Zealand for a year and i discovered the best shampoo ever for white horses ! I'm very impressed. It's called "Naturally white" you can find it on this web sitehttps://www.beekind.co.nz/categories/Bee-Kind™-Equestrian-Products/
You can see more photos on the Facebook page. https://www.facebook.com/BeekindNZ/ if you want. Hope it's going to be helpful for you :)
I dont know what its called but its purple shampoo and it works amazingly!!!! I used it on my old horse and he always glowed when he wore it, I also had to use it because he was allergic to the other kinds of shampoo 😂😁
I personally use the @ravene shampoo Easy shine grey. It smells good and really helps removing yellow stains from the horse coat. It leaves the hairs very soft to the touch.
I've also heard good things about the gallop shampoo for grey horses.
Hope it answers your question :)