Dog whitening shampoo! It's safe and works AMAZING! I used to have an appy who was white with brown spots and that helped soooooo much
If seen people use baby powder
I've seen people use baby powder or coconut oil not mel
But just shower the horse everyday and brush and currycomb it properly everyday. In at most a month I think it will be gone, with no odd experiments
I once used ash as a "shampoo" before applying the regular shampoo. And I lately heard about ketchup applied on stained tails for no longer that 15 mins to get a whiter tail (more that that might get you a pink hue - fancy!). Never tried the second one though...
And make sure to condition afterwards because it seals the hair and prevents stains from penetrating
Scrub with a purple whitening shampoo and let it sit for 5-10 minutes before rinsing, make sure to get deep down when scrubbing