Hi Rafael
This might sound simplistic but rather than giving actual colour refs
1-mix or find a base colour that can be darkened or lightened but
will essentially compliment the oridginal mix. You need this for graduation
2-Apply this base colour to the facial areas, next transfer some of the base
colour to another well on your pallette and darken with a mid brown.
Apply to all shadow areas (note this colour must be thinner than the base colour)
3-You should have a face that is begining to have some shape.
Take some more base colour and lighten it with some off/white (white+yellow is ok) apply this to the higlight areas.This gives form to the face.
4-Now study your work so far, there will be areas where the transition between
light and dark produce a definite line. This can be corrected by mixing a tiny amount of shadow colour with the highlight colour. This will blur the line between the two. If your highlight appears too bright, some very thinned down base colour
will pull it back.Hope this is of some help to you, just ask if you need any more
Frank