Vermeer produced transparent colours by applying paint onto the canvas in loosely granular layers, a technique called pointillé (not to be confused with pointillism). No drawings have been securely attributed to Vermeer, and his paintings offer few clues to preparatory methods. David Hockney, among other historians and advocates of the Hockney-Falco thesis, has speculated that Vermeer used a camera obscura to achieve precise positioning in his compositions, and this view seems to be supported by certain light and perspective effects which would result from the use of such lenses and not the naked eye alone; however, the extent of Vermeer’s dependence upon the camera obscura is disputed by historians.
There is no other seventeenth century artist who from very early on in his career employed, in the most lavish way, the exorbitantly expensive pigment lapis lazuli, natural ultramarine. Not only used in elements that are intended to be shown as appearance: the earth colours umber and ochre should be understood as warm light from the strongly-lit interior, reflecting its multiple colours back onto the wall.
This working method most probably was inspired by Vermeer’s understanding of Leonardo’s observations that the surface of every object partakes of the colour of the adjacent object. This means that no object is ever seen entirely in its natural colour.
A comparable but even more remarkable yet effectual use of natural ultramarine is in The Girl with a Wineglass (Braunschweig). The shadows of the red satin dress are underpainted in natural ultramarine, and due to this underlying blue paint layer, the red lake and vermilion mixture applied over it acquires a slightly purple, cool and crisp appearance that is most powerful.
Even after Vermeer’s supposed financial breakdown following the so-called rampjaar (year of disaster) in 1672, he continued to employ natural ultramarine most generously, such as in the above-mentioned “Lady Seated at a Virginal.” This could suggest that Vermeer was supplied with materials by a collector, and would coincide with John Michael Montias’ theory of Pieter Claesz. van Ruijven being Vermeer’s patron.
(Taken from the Wiki)