I had a wheat colored wool that I dyed with Rit "Dark Green" --- it came out grey. Not a touch of green at all.
It CLAIMS to work on cottons and wools, but I won't use it again.
"Choose your friends by their character and your socks by their color. Choosing your socks by their character makes no sense, and choosing your friends by their color is unthinkable." http://RoseByAny.BlogSpot.Com
There is nothing as good as Rit dye for tie dying cotton t shirts. I've also used it successfully on white cotton nightgowns to change to solid colors, as well as cotton terry towels. They actually make tons of colors and you can control the result of how light or dark you want each color.
I haven't used Rit in a while though. I always used it in a top loading washing machine with an agitator but I don't have that kind of machine anymore. I would only use it on 100% cotton, regardless of what the pkg says about wool.
I experimented with RIT in high school on cottons and found that although the colors came out well, they faded terribly and after 6 months of wearing and washing, I had a nice off-white for re-dying. I prefer Procion dyes for cottons, even though you have to use the yucky toxic powders. At least they're permanent! Lin