“[C]olor itself is capable of two very different kinds of relations: relations of value, based on the contrast of black and white, in which a tone is defined as either dark or light, saturated or rarefied; and relations of tonality, based on spectrum, on the opposition of yellow and blue, or green and red, in which this or that pure tone is defined as warm or cool. . . .
     Are there not two very different kinds of gray, the optical gray of black-white and the haptic gray of green-red?”

—Gilles Deleuze, Francis Bacon: The Logic of Sensation, 1981; trans. by Daniel W. Smith, 2004.