If you wanted to paint it, some makers still just use watered down acrylic paint. And since the head usually just gets touch-ups and not a full blown water washing, it could work pretty well. Like Alistar said, using a hooked blade or an exacto knife would be the next option, and use a blanket stitch or similar to sew the markings into the head. It's okay if the foam tears a bit, since the fur will be sewn together and will hold it in place.