I have never had much luck with hair dye. Leather dye is a possibility, and Rit dye is an old stand-by that many horners use. The trick is to find some colorant that has some wear resistance to it. I personally think that potassium permanganate, which many people recommend, doesn't produce a believeable color and is a surface dye at best.
Acid dyes can be used to produce a nice color on horn material Aqua Fortis produces a brown color on horn. Copper dissolved in nitric acid produces a green dye and silver or nickel dissolved in acid produces a black dye.
Horns can also be dyed in a hot walnut hull bath. Or simply rubbing fresh walnut hulls on a horn will give a brown color.
There are so many things that will dye a powder horn, you just have to experiment to get a color you like.
One thing that will help preserve the color on any dyed horn is to give it a couple of good coats of paste wax meant for automobiles or hardwood floors. After that, apply a coat of wax every once in a while.
Randy Hedden
www.harddogrifles.com