thanks for the comments! :-)
@Peter: the technique I use is called pin wash. I use oil paints for this ( Van Dyck brown in this case) Dilute the oil paint with White Spirit (10% paint, 90% white spirit) Use a very fine brush and load it with the diluted paint. When you touch the part you want to accentuate lightly, the capillary effect will let the paint flow around the part. Don't use too much paint though. Let the paint dry for about 10 minutes. By now it will have feathered out a bit. Now we take a clean fine brush and we dip it in clean white spirit. the brush needs to be damp, not wet. Now clean up the line around the part until you are satisfied with the effect. Sometimes you'll have to go back and re trace with the paint/white spirit mixture. it might sound a bit elaborate and it takes a bit of time, but the effects you can achieve are well worth it.
@ John: don't know if it's pure spanish style, but there are some influences for sure :-)
@ Dave: thanks a lot for the parts!! Much appreciated!