I'm doing a panel this weekend at a local con for dyeing wigs with RIT. I threw in some other samples for fun.
These are all done in the same dye bath.
http://www.cosplay.com/photo/3045091/
...and here is a photo of Kelly Green RIT versus what you get out of the box.
http://www.cosplay.com/photo/2924951/
I've also ordered from Dharma Trading company. They sell a great silk / cotton blend. It's great for dyeing. It has the sheen of silk but the durability and stability of cotton.