The Neural Control of Vision
J. The Processing of Color

The first step in trying to understand color is to come up with some clear rules about it. We distinguish three attributes of color: hue, brightness, and saturation. Newton had worked on the rules of color and had come up with the color circle that was subsequently modified and extended. Figure 43 shows a recent version of it. The white oval contains colors that are all equally bright (isoluminant), in which the central point is white. Opponent colors are at opposites along the circle. Color saturation increases from center to periphery, and hue varies around the circle. Brightness is along the third dimension with the top being bright and the bottom being dark. The rules that derive from this arrangement are powerful. According to Grassman's laws, every color has a complimentary which when mixed properly yields gray; a mixture of non-complimentary colors yields intermediates. Another important fact is that when the color circle is set up according to the sensitivity of the cones, it can perfectly predict the formation of afterimages.