If you do use the potasium permanganate, be sure to follow Lin's advice and wear rubber gloves. I had to clean up some that had spilled a few years ago and didn't know it would stain like it does. I was sweating a lot and wound up with purple hands and purple spots all over my arms and legs for a week. Boiling walnut hulls gives a nice dark brown die for leather, but I don't know how it would work for bone/antler/horn. A torch works real good if you're careful.
Todd