Paint Lights More Thinly on the Edges

"The advantage for painters of using a dark preparation is that its colour can stand unmodified for many parts of a composition. It can be used for shadows, or can be darkened further with a transparent glaze for deeper shadows.

"...by painting the lights more thinly towards their edges, allowing the dark ground to show through, cool half-tones are automatically achieved."

-- Velázquez's painting technique described in a treatise written by his teacher and father-in-law, Francisco Pacheco, in 1647. Quoted in Colour: A Closer Look by David Bomford and Ashkok Roy, National Gallery, London p82 (Buy Direct)