Here's a sample. This is not a full blown good skin shader - just the essentials to show how to use a hair map.

My hair map is from Apollo, and it is black on white. Actually it is gray on white. Anyway, I inverted it using Math:Subtract to make the hairs lighter than the black where skin should be free of hair.

Plug the hair map into the Blender Blending value. Put the skin color in Value_1 (I used an the Apollo color map) and then hair color in Value_2 (I used a fixed color - brown).

The Blending value can be any number you want, and the higher it is, the more the hairs are visible. You can see a render and some previous renders at different blending values.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)