I have dyed hemp with walnut hull dye and not had a whole lot of success with getting the color I want. A traditional dye expert recently told me that walnut hull dye works best with animal products, and not with plant products. I believe it has something to do with the acid and or tannins in animal products which are not present in most plant-based fabrics.
Common Rit Dye will work fine. I also use diluted Fiebings leather dye and have had great success with that. It doesn't seem to fade, even after several "washings."
I suggest experimenting on a small piece of scrap, if you have one, and going from there.
Good luck,
Jeff