Fig. 1.

Color opponent axes. Hering declared red and green to be opponent colors represented along a single axis. Here, positive values on the axis are reddish, negative values are greenish, and zero represents a hue that has no redness or greenness at all—that is, unique blue, unique yellow, or white. Hering also proposed a second axis running from yellow (positive) to blue (negative), with zero for hues that have no yellowness or blueness (so unique red, unique green, or white). Any hue can be represented by two coordinate values, one on each axis. Orange, for example, has a positive value on both axes; aqua, composed of both greenness and blueness, has a negative value on both axes. White has zero on both axes (no trace of redness, greenness, yellowness, or blueness).