fabric softener
I haven't ever seen problems with FS and my towels. My towels are dark blue and get washed with other dark items like jeans, shirts, socks, etc. I use warm water, a good detergent, and ammonia. I assume that this removes the previous dose of FS along with dirt and stains.
I imagine that most people who have problem with their towels wash in cold water and use way too much FS.
David