A thin, lighter layer of semi-opaque color placed over a darker underpaint. The presence of the darker underlayer will, as a rule, shift the appearance of the scumble towards a blue or cooler tone that leads to a dulling of the tonal color effect.

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 62.