Hi Adriano,
I really like the depth created between the girl's face and the background in the first image of the second post. The contrast between the detail of the face and the looseness of the background is great. It has a similar effect as Pino Dangelico's work (
example ). It might be worth exploring the idea of letting some of the pastel show through in the loose areas.
I was told by an art professor that using bunt umber (or similar color) for the under painting helped to bring out a "realistic" depth to the layers of skin. This is assuming that the layers of oil on top of the under painting are not overly thick of course. I'm not sure how true it is, but I would guess that the color of the pastels might play a role in this as well. It would be interesting to do a comparison between the two methods.
Justin
Edit: I just realized that the year on this post was 2006. Please forgive my newness.