I suppose it might work on cotton. i've only ever done it to wool. And so far i have had no bleeding or dye come off on my hands while spinning. I've yet to make and wash anything from all this yarn I've dyed with wiltons. It takes a few good rinses to most of the dye out, and even the stuff I gave up on (getting all the dye out) still does not leave any color on my hands. Oh and yes. White vinegar. my ratio of 50/50 vinegar and water is overkill. you can get away with 25/75 I believe. i've also used coffee and red wine vinegar, you get a redish coffee stain type brown when you do that!