An excerpt from Velazquez The Technique of Genius
by Jonathan Brown:
Velazquez does not seem to have started with a fixed idea for a composition but rather preferred to see what happened as he worked, making adjustments as he painted. in many pictures, these modifications are readily seen with the naked eye. The contours of figures overlap as their position in the composition changes or as elements are added or subtracted. Even within the forms of individual figures changes can be observed. The positions of hands and sleeves are adjusted, collars and lace are shifted, as are other parts of costume.

Landscape and neutral interior backgrounds were added, generally speaking, after the contours of the figures had been established. This practice is most evident in paintings done before the Forge of Vulcan (Plate 5), when Velazquez frequently outlined the figures in black or white, separating them from the background. Fine white lines, which are visible in raking light and radiographs, define the contours of the figures and the shape of the fabrics. These lines, which have a certain relief, were created by the accumulation of pigment along the edges of the brush. in the later works, these contours tend to dissolve, although from time to time they are re-introduced.

Various concentrations of pictorial matter are visible in different parts of the composition, notably in the transitions between figures and background. Where compositional changes have been carried out, or where figures overlap, the density of pigment is greater and has more relief. One of the most interesting idiosyncrasies of Velazquez's technique is the use of blue pigments in combination with whites, ochres, and yellows to create the greenish parts of a painting, be they landscapes or costumes. The entire range of blue in the skies and of green in the vegetation is achieved by mixing these pigments. A green pigment of the kind typically used by other painters of the period never found a place on VelAzquez's palette.