This chapter explains the mapping of color and other nonspatial channels in visual encoding design choices. The colloquial term color is best understood in terms of three separate channels: luminance, hue, and saturation. The major design choice for colormap construction is whether the intent is to distinguish between categorical attributes or to encode ordered attributes. The retina of the eye has two different kinds of receptors. The rods actively contribute to vision only in low-light settings and provide low-resolution black and white information. The visual system immediately processes these signals into three opponent color channels: one from red to green, one from blue to yellow, and one from black and white encoding luminance information. The luminance channel conveys highresolution edge information, while the redgreen and blue-yellow channels are lower resolution. This split between luminance and chromaticity--what most people informally call would normally call color--is a central issue in visual encoding design.