My guess is aqua fortis blushed with a heat gun or old timey soldering iron. Brush on and use medium heat to bring out the golden tones. KEEP THE HEAT SOURCE moving! If you don't like the result, sand off and try it again.
You see this coloration on lots of period horns such as Tansels.
Look at Art DeCamp's website and you will see this color on some of his horns.
Take Care,
Rick Sheets