Several systems have been devised to measure colors. For most of us not directly concerned with dyes, paints, or inks, the subject was academic until recently, but now computers require precise methods for describing the colors to be displayed or printed. These methods typically use three variables, reflecting the fact that the human eye has three types of color sensor. Computer monitors use the RGB system, which specifies colors with three variables measuring the intensity of the three primary colors red, blue, and green in the color. Frequently each variable is specified by one byte and therefore takes values in the range 0 to 255. If all three are 0, the resulting color is black; if all three are 255 the resulting color is white. The RGB settings for the Carolina blue background of this page are R=153, G=204, and B=255. Since it is difficult to estimate the relative amounts of red, green, and blue needed to create a particular color, many graphics design programs use the HSV color system, which describes colors using three variables called hue, saturation, and value. Once again, all three variables are assigned values from 0 to 255. Hue, which is what we call "color" in ordinary language, is described on a circular scale. Hue values begin with red at 0 and run through yellow, green, blue, and purple before returning to red at 255. Saturation is the purity of the color, the extent to which it is not watered down with gray. The pure color has saturation 255. As saturation is reduced, the color becomes grayer, until at saturation 0 the color is replaced by a neutral gray of the same intensity as the original color. The value (or intensity) of the color is its brightness. The pure or most natural form of the color has value in the middle of the scale, at 127. As value is increased the color becomes brighter. In the opposite direction, the color becomes less bright, becoming black at value 0. This page's background has hue 140, saturation 240, and value 192 in the HSV system. See also: Lovibond color units (used for beer and honey)