One of the special things that characterise High Renaissance paintings is a smoky appearance - as if a drift of thin, fine, net curtain had been drawn across the paintings. The darks seem slightly lighter and the lightest places somewhat duller. The master of this (and almost everything else!) was of course Leonardo da Vinci.
The Italian word for this effect is Sfumato, from the word sfumare, 'to smoke'. Look at those wonderful soft corners of her mouth - that subtle shading that, from a distance is partly the reason it is difficult to tell if she is smiling or not. I once read that she was an intelligent woman. The mouth would suggest that she found da Vinci interesting. Or at least, that what I'd like to think. That he was propounding something enthusiastically to her as she sat, and he was challenged to capture that gentle amusement at his excitement.
Anyway, sfumato - the smoky look. Not that hard to do, actually. Very thin titanium white is the secret. And lots of glazing in layers.