All of the mentioned inks and mixes work well, as does artists acrylic, which allows you to swing from black to brown -depending upon horn color and the specific look you're after.
If you're concerned about the plastic look that often comes with pre-polishing the horn to the point where all random, ink-grabbing scratches, scrape marks or surface texture are eliminated, try the following:
Prepare the horn surface to your satisfaction; draw your scrim lines/images with a soft, sharp pencil; spray the entire scrim area with aerosol hair spray [most is water based]; do your scrim; darken the scrimwork with the substance of choice -carefully wiping off the excess. When you're satisfied, wipe down the horn with a water-dampened cloth, which removes the hair spray while leaving the color where you want it.
The hair spray also allows you to more comfortably handle the horn during the scrim work, without smearing the image.
~Longshot