Let's be honest, this isn't really a tutorial as much as it's "how I do it." When I posted the Terminator Librarian, I got a couple requests for how I painted his face and more specifically, his beard or stubble more accurately.
It's a real simple process.
1. Prime black
It's the color I use most of the time.
I use a light tan for the skin and silver for the metals. I don't paint eyes and I don't paint teeth. I can't see them at 3 feet away and they're on the side of the model I don't see during the game anyway.
I also leave a thin line of black between colors. It covers any bleed over from the washes and adds to the depth of the face where skin meets metal.
I use a Ogryn Flesh wash on the skin and Badab Black on the metal. Heavy on both.
4. Touch up basecoat
I'll go back with the original base colors and add them back to the raised areas like highlights. Since I went heavy with the wash, there's enough contrast to pull this off. If I want to add a second highlight, this is when I do it, but only to the most raised portions on the face.
5. The stubble
I don't paint heads any different if I'm going to add this step or leave it off. Now is when I add the stubble effect.
I use a medium shade of grey, it's a warm grey but I don't think that matters really. The big question you have to ask yourself is how long ago did he shave? I thin my paint down just a little bit and paint it right on the face where he would have the start of a beard.
Simple as that.
One thin coat = one day growth
Two thin coats = two days growth
After that, you might as well whip out the Greenstuff and add his beard on there.
The last step actually is to go back with a thin Devlan Mud wash and touch up just the recessed areas in the stubble (and the eyesocket for added contrast). This adds the depth back into the face that might have been erased when you were adding the stubble.
And the same thing applies for shaved heads...
Hope this helps, it really is simple to do and it adds a bit of realism. Thankfully, a friend of mine gave me 5 Space Marine heads that I could use to do a bunch of tutorials like this, so look for more in the coming weeks.
UPDATE: For those looking to go the extra distance, this post on sculpting facial hair might help.