Horse tail still yellow?!?

I showed a grey horse for a couple of years. He was one of the dirtyest horses I've ever worked with. Before shows I take Quic-Silver, and mix it with Jeffer's Brightening Shampoo and put it into the tail first. Mix with a little bit of water, and get it really soapy. Run it through the tail first, then bath the rest of your horse. I like to leave the mixture in the tail for 5 minutes or longer. Rinse. After its dried I spray show sheen, braid the tail and then put it in a tail bag.