Unique hue is a term used in certain theories of color vision, which implies that human perception distinguishes between "unique" (psychologically primary) and composite (mixed) hues. A unique hue is defined as a color which an observer perceives as a pure, without any admixture of the other colors. There is a great deal of variability when defining unique hues experimentally. Often the results show a great deal of interobserver and intraobserver variability leading to much debate on the number of unique hues. Another source of variability is environmental factors in color naming. Despite the inconsistencies, often four color perceptions are associated as unique; “red”, “green”, “blue”, and “yellow”.