The Natural Color System (NCS) is a proprietary perceptual color model. It is based on the color opponency hypothesis of color vision, first proposed by German physiologist Ewald Hering. The current version of the NCS was developed by the Swedish Colour Centre Foundation, from 1964 onwards. The research team consisted of Anders Hård, Lars Sivik and Gunnar Tonnquist, who in 1997 received the AIC Judd award for their work. The system is based entirely on the phenomenology of human perception and not on color mixing. It is illustrated by a color atlas, marketed by NCS Colour AB in Stockholm. Definition The NCS states that there are six elementary color percepts of human vision—which might coincide with the psychological primaries—as proposed by the hypothesis of color opponency: white, black, red, yellow, green, and blue. The last four are also called unique hues. In the NCS all six are defined as elementary colors, irreducible qualia, each of which would be impossible to define in terms of the other elementary colors. All other experienced colors are considered composite perceptions, i.e. experiences that can be defined in terms of similarity to the six elementary colors. E.g. a saturated pink would be fully defined by its visual similarity to red, blue, black and white. Colors in the NCS are defined by three values, expressed in percentages, specifying the degree of blackness (s , = relative visual similarity to the black elementary color), chromaticness (c, = relative visual similarity to the "strongest", most saturated, color in that hue triangle), and hue (Φ, = relative similarity to one or two of the chromatic elementary colors red, yellow, green and blue, expressed in at most two percentages). This means that a color can be expressed as either Y (yellow), YR (yellow with a red component), R (Red), RB (red with a blue component), B (blue), etc. No hue is considered to have visual similarity to both hues of an opponent pair; i.e. there is no "redgreen" or "yellowblue". The blackness and the chromaticness together add up to less than or equal to 100%. The remainder from 100%, if any, gives the amount of whiteness (w). Achromatic colors, i.e. colors that lack chromatic contents (ranging from black, to grey and finally white), have their hue component replaced with a capital "N", for example "NCS S 9000-N" (a more or less complete black). NCS color notations are sometimes prepended by