I was reading on the web about staining horn and to get the dye to stay it was boiled, the water(and dye) is then drawn into the expanded micro pores, when it dries the pores close and keep a little dye on the surface. Maybe a strong pink food dye in boiling water for a while would work?
(Obviously when the horn is blended flush at the base with the bow, and string groove cut that will remove the dye, but might contrast well with the remaining pink?)
(Another idea is acrylic horn blanks used for making pens etc. They come in all colours and cost about a fiver tops, I've seen them used for nocks.)
R.D.