Can I assume you are wanting to know how to "Yellow" a new horn to look old?
The Sibley's post 3 methods to do this:
1. Use Aquafortis, a diluted nitric acid and iron compound, and apply it to the horn. Then use a heat gun to warm the surface until it turns golden.
2. Use Onion skins and apple vinegar , mixed with water. Put mixture in water and heat to a boil. Turn down the heat and put the horn into the mix, checking the color until you get what you desire.
3. Use Golden Yellow RIT dye- 2 boxes and enough water in a turkey cooker to cover the horn. Boil. Try this first with a junk horn, to check the color. He says he likes to put a "Pinch" of orange dye in the brew to get a darker yellow color. He also puts the base plug into the horn before dyeing it. He sometimes paints the base plug before dunking the horn in the mix, and the hot dye bath will cause the paint to lift, giving the plug an antique look.
To Add false Patina, after the horn is scrimshawed, apply dark stain- he mentions powdered tempura paint-- to the horn and rub in . He also suggests walnut stain- oil based, to darken the horn. If you want the horn to seem used and beat up, it should be distressed, with scratches, nicks, etc, before applying the " patina". Rub as much or as little of these stains on as you want. He does not use India Ink to highlight the scrimshaw if he is "antiquing" the horn and adding patina. Either the black tempura dye powder, or the walnut oil stain will darken the scrimshaw enough to be easily seen through the "Patina".
I hope that helps.
Personally, Unless the maker clearly marks his horn with name and date( in several places, both inside and outside the horn), this kind of thing is likely to defraud some unknowing buyer in years to come, thinking they are paying for a hundred-years old horn, when it in fact was recently made. I don't like the practice, unless the safeguards are followed. It takes a very experienced and knowledgeable horn builder to determine the age of any horn, short of very expensive and sophisticated scientific testing, which most people can't afford, and have no idea where they can get it done.