Go to laundry section of store and look up on top shelf, which is where it usually is, Mrs. White's Bluing. Read directions on it, you have to dilute it, and make sure you hold tail away from body to dip, as if you have it too strong, it will stain horse too, although it comes out. But dipping in it will whiten it.
Can also use White Mink, which is in hair dye sections, and is a rinse that whitens too. May be called White Minx, been a while since looked, but bottles again, usually on top shelf or thereabout, bottles are grayish/gold, and used to have pink applicator tops. Wash tail, rinse good and then apply this. Rinse out after working it in good, and dry and wrap.
Horses make me a better person.