If he has silver shot through his tail? You can wash/rinse it in White Mink shampoo for people, it is usually at wrongmart on top shelf, and is in grayish bottles with pink tops, and is for women who have gray/silver hair and helps to take the yellowish tint out of it. Doesn't dye it per se, just brightens the silver strands.
I used to use it on WC French Commander when I groomed Saddlers. It also seemed to leave the hairs so that when fingerpicked out after washing, they were more "separate" and left a really full tail.
It really made his tail, which was tremendously long anyway, glisten in the lights of the ring.
But, if your horse is in training, the grooms should be doing all of this, not you, unless you are grooming for training barn your horse is at? Trainer may not want you messing with groom's business?
Horses make me a better person.