It's inevitable if you have a light colored horse you've dealt with the problem of dingy, yellowed tails. Unfortunately, ALL white tails will yellow, it's their natural color. A Lipizzaner breeder once told me that once the hair is a certain length it's actually dead at the ends and will turn yellow no matter what you do.
The cells of a white tail are are "clear" and since hair over a few inches is actually dead, the dead cells when they show light through them have a yellowish tinge.
The good bluing shampoos work because the bluing is actually a kind of dye that counteracts the yellow (doesn't remove the yellow, but tints it bluish, which the human eye sees as white) I always use Fiebing's Blue Frost Whitening Shampoo & Conditioner on my grey and white horses. In between baths I try to use a blueing spray, which helps with day to day control of yellowing tails.
Unfortunately there's no bluing spray on the market currently but I recommend 2 tablespoons Mrs Stewarts Bluing added to a bottle of Infusium 23 leave in conditioner. This conditions manes and tails while gently whitening.
Braiding your horses tail into a Tail Bag can help prevent staining from environmental factors. Please carefully consider when to braid tails- we only leave tails braided in the winter months when tails aren't needed for fly swatting. If you use a tail bag, unbraid the tail, wash it with whitening shampoo, and rebraid it every 2 weeks or so, and do a hot oil treatment every 6-8 weeks.
These pictures show the results of my tail yellowing remedies on one of my horses: