Hue is one of the main properties (called color appearance parameters) of a color, defined technically (in the CIECAM02 model), as "the degree to which a stimulus can be described as similar to or different from stimuli that are described as red, green, blue, and yellow" (the unique hues). Orange and violet (purple) are the other hues, for a total of six, as in the rainbow: red, orange, yellow, green, blue, violet. The other color appearance parameters are colorfulness, chroma, saturation, lightness, and brightness.

Hue is the gradation of color.