While I agree that the antler buttons look best natural, I'll suggest trying Fiebings Leather dye, applied carefully. I've used it to good effect on horn, bone and antler. It comes in a wide range of colors, although anything beyond black and brown will likely require finding a leather craft supply. Black and brown can usually be found in shoe stores. One final caveat, wear gloves. Once the dye gets on your skin, it pretty much has to wear off.