Dharmatrading.com. If your towels are cotton, you want Fiber-Reactive Dyes. The color actually becomes part of the cotton fibers, rather than sitting on top of them like Rit. I have t-shirts that are 8 years old, worn and washed regularly, and while *I* can tell that they've faded a bit, no one else can. Still bright and new looking.
If you find that you can't make a decision on which dye you want, contact them. They have always been extremely helpful.