Hi Peggy,
In Harold Speed's "Oil Painting Techniques and Materials",
http://www.portraitartist.com/bookstore/oil.htm on page 117 there is a reference to Vermeer.
Speed writes, "A movement of colour in the tones, such as the impressionists developed to such an extent, is necessary in all vital colouring; and has always existed even from the days of the tempera painters. They painted one colour thinly over another repeatedly, thereby getting a play of two colours. Their flesh was usually painted in terra verte in the first instance, the pinks and ochres being thinly painted upon it afterwoads. There is evidence that Vermeer of Delft used this method. In "The Lady at the Virginal" already referred to, the terra verte underpainting shows through in the head of the girl very strongly. The flesh tones that had been thinly painted over this, have been rubbed off, I imagine, in the process of cleaning the picture at some time."
If you know of another good source on Vermeer would you please share it with me?