I come at it kinda "backwards," having learned the stunt in our brandings. We always seem to come up with a few heifers from the sales with horns (though too small even for priming horns). No answer for it but to take them off, which means using the big dehorning nippers, then cauterizing. Lots of blood, plenty of which gets on the horns. Any of those that don't get carried off by the dogs go into the bone yard.
I noticed some of these months later after a long period sitting in the sun, then being rained on, retained a really pleasing color wherever they had blood on them. I 'sperimented with rubbing blood on another horn from the yard, then letting it sit a few days before washing and waxing. It looks good enough that I'm going to try it on a build next time I get the chance. Probably look funny carrying a finished horn to a branding, but that's the best source I know of. Might do just as well with blood from a tub of beef liver from the market though. Can't say for sure how well it will wear till I play with the idea more, but even if the color wears through here and there over time, I'm betting I'll like the results. Not really "yellow" yet, but maybe that will happen years from now. In the meantime, it sure looks good to my eye. Sorry no pics.