Mi Michael, well at 1/35 scale you can get away with painting certain details but if you want to show, say, a week's growth then I'd probably add real texture myself, even if only on the chin, upper lip and the cleft under the bottom lip where hair growth can be faster.
I might do this by stippling thick paint on, this gives quite a good representation and is easy to control, especially in oils. Or you could 'paint' it on with a slurry of putty in much the same way - just add a drop of water to a little freshly-mixed putty and wait a few minutes for it to be absorbed - I've used this method once or twice on my sculpted heads for shaved areas that are growing out.
If you use paint I would do this over the primer and then undercoat the area in the skin basecoat, it's probably best not to try to do this directly with the beard colour. For putty I would do it on the metal/resin and then prime over it but you can make this kind of addition over primer without problems if you've primed already.
Einion