I've used RIT dye, with mixed success. I would only recommend dying clothes that you won't be devastated to part with. Be sure to get the garment COMPLETELY wet in regular water before putting it in the dye. No matter how careful I am and how soaked I think something is, I usually find out that part of it wasn't quite as wet as the rest and I end up with a couple areas that aren't as well dyed as the rest.
Good luck, and let us know how it turns out!