Another thing that will work well, and it's inexpensive is Dawn Dish soap.
Yeah, I said it. Dawn.
I had a grey Arab gelding, and he was famously dirty. His tail was white, but I thought it would always have a yellowish cast to it. I used Quicksliver, bluing, anything.
Well, I wanted him sparkling clean, and so I put his tail ends in a bucket filled with dawn dish-soapy water, scrubbed it, washed it with dish soap straight, and then rinsed like the devil.
That tail was so sparkling white, I was floored. He looked gorgeous. Just make sure you use spray on coat conditioners (I prefer Laser sheen) to condition the tail hairs, and to prevent further staining.