Purple is a general term for the range of shaes of color occurring between red and blue. It is formed (in both subtractive pigment and additive light combinations) by mixing the primary colors red and blue in varying proportions, with possibly a very small quantity of the third primary color (green for light or yellow for pigment). There is disagreement over exactly which shades can be described as purple, some people preferring more precise terms such as magenta or heliotrope for particular shades. A difference in retinal sensitivity to red and blue light between individuals can cause further disagreement.