Brian's technique should work a treat if you want to work wet-on-wet. Colour-wise, for someone with very dark hair you can use just a plain black + white mix but sometimes this will look too blue, if you get this try adding in a tiny amount of umber. For people with lighter hair I usually use a mix of Titanium White and either Raw Umber or Burnt Umber depending on whether they're more blonde or more brown.
If you'd prefer to apply the beard shadow in a fresh layer so that it's a little more controllable (you can wipe off and start again if necessary) you can use the same mixtures as above but it's sometimes better to blend them with a little of your flesh mixtures beforehand.
Einion