I've actually heard that tomato sauce does great for white tails because of the acidity levels or something. My friend has a cremmello horse and when it's dirty she'll always put tomato sauce in the tail and it works wonders :)
For anything white both coat and tail: Orvus paste. It always gets the job done and comes in big tubs so you won't run out any time soon
My trainer uses Dish soap! Ketchup works to haha I know it's not a product but they work!