Answer:
Da Vinci's most common style was oil paint, he utilized oil paints and the sfumato technique to create the image of Mona Lisa.
The technique, Sfumato (s-foo-mado), is just allowing the tones and colors to shade into each other, this creates soft and hazy outlines.
Da Vinci did was made his under painting by adding the tint and tone, white and black, to pure colors. He wanted them to look real.
If you talk to the right crazy artist, they'll share their own spectacle of how he'd paint his own face into the underpainting first, it's quite a joke.
