Thanks again everyone. I hope to do more like this one with the fun, folk-art motifs and colors.
Flatsguide, the colors are waterproof India Ink and they seem as permanent as standard Black Ink. I tested some on a spare piece of horn and they seem very stable. In addition, all my decorative work is engraved into the horn, not just applied to the surface, so the colors are in the inscribed lines so they won't wear away. Paste wax is applied and seals it pretty well.
Thanks for asking !