A technique in which you juxtapose warm/cool temperature variations of a color that are slightly more saturated than the final saturation you are aiming for. At a distance, the eye will blend the warm/cool color spots into the intermediate hue and lower the saturation of the resulting mix to the correct saturation you were aiming for. Because the eye is doing the mixing, more light reaches the eye than if you were to mix the colors physically on the painting. This is what gives the painting luminosity.