Hey! My mare is a bay, but one of my closest equestrian friend’s horse was a white gelding and he loved to roll. My advice is after a bath using bluing shampoo (this will give the hair a silvery cast) spray the coat all over with ShowSheen or a similar product to help repel further stains. Also if you don’t have a time for a full bath you could use water-less spot cleaner to remove the mud so it doesn’t totally stain his coat. another version of this is to make a paste of bon-ami cleanser and water. I don’t remember the recipe fully but you can probably google it. Hope this helps!