I haven't even completed my first horn yet, but I wanted to share/ask. I see most people use rit.
I used Revlon bright red hair dye in a tube. Before I knew about rit, I figured hair and horn are pretty much the same. Has anyone tried this before? I got great results. It's about toothpaste consistency. I mixed it with a bit of peroxide into more of a yogurt consistency and painted it onto my horn. Wrapped it in saran wrap for 30 mins and the result was a vibrant red. Abomination, I know, but it is what I was going for. I probably have enough to do 20 or 30 more horns at least.
I was making progress with the horn, until I turned the beehive bung and discovered my wood had beetle damage and found signs of an active infestation. That derailed my horn project. I've been treating everything in my garage/shop (which is a lot of lumber, bench, cabinets, jigs, wooden planes, saws, chisel handles, mallets, ....) but I hope to get back to it in the next few weeks.