# NCS - Natural Color System

The four NCS chromatic elementary colours(red, blue, green and yellow) are placed 90 degrees apart on the hue circle and form the basis of an opponent axis system with the red-green and blue-yellow elementary colours opposite each other. The two achromatic elementary colours (white and black) are also opposites, and are situated vertically above and below the centre of the hue circle. Absolute white is placed the same physical distance vertically above the centre point as absolute black is placed vertically below. The physical distance between the two achromatic elementary colours is the same as that between either of these elementary colours and the arc of the hue circle containing the four chromatic elementary colours. Thus NCS colour space is a regular solid which resembles two cones attached at their bases and an NCS constant hue chart is an equilateral triangle with absolute black (S), absolute white (W) and maximum chromatic colour content (C) at its three points.

The four chromatic elementary colours are also divided into 100 step scales, producing 400 steps around the hue circle. NCS hue is expressed in degrees of resemblance to one or two of the chromatic elementary colours, Y, R, B and G. A hue which resembles only one of the chromatic elementary colours is expressed as that elementary colour (e.g. Y): hues which have resemblance to two of the chromatic elementary colours are expressed as a percentage of the visual content of those two elementary colours. A brown/orange hue has the notation Y30R, which indicates that it resembles yellow 70% (100-30) and red 30%; the sum of the two chromatic elementary colours always being equivalent to 100.

Each of the elementary scales are divided into 100 steps. S = 0 with W = 100 is equivalent to absolute white and S = 100 with W = 0 is equivalent to absolute black. Constant blackness is represented by diagonals sloping downwards parallel to the line joining W and C: constant whiteness is represented by upward sloping diagonals parallel to the line joining S and C. NCS chromaticness (C) extends from the central (achromatic) axis, C = 0, to the hue circle (maximum chromatic colour), C = 100. Constant chromaticness is represented by the lines parallel to the central axis. The relationship between the white, black and chromatic coordinates is such that their sum is equivalent to 100.

blackness (S) + whiteness (W) + chromaticness (C) = 100

Thus the blackness, whiteness and chromaticness of a particular colour represent percentages of resemblances to black, white and maximum chromatic colour. The plane of constant NCS Chromaticness of 50 is given below.

The plane of constant NCS Blackness 40 is given below.

The NCS notation to uniquely identify a colour in NCS colour space is thus blackness chromaticness - hue; SC-H. For example, a colour with Blackness = 20, Chromaticness = 40 and hue R90B is expressed as 2040-R90B. The NCS method for the specification of colour was adopted as a Swedish Standard in 1972-1973, and is available from the Swedish Standard Institute as Svensk Standard SS 01 91 00E (1978).