The colours of the RAL DESIGN System are organised systematically in Hue, Lightness and Chroma values. Fig. 1 shows the spatial building of the RAL DESIGN System. Following the sequence of the colour spectrum, the Hues are organised in a circle. The designations correspond with the angles. Red can be found at 0°„ (= 360°„), yellow at 90°„, green at 180°„ and blue at 270°„ ( fig. 4 ). The different values of Lightness that are possible within each Hue are arranged in various levels. Fig. 2 shows one of these levels. The non-chromatic axis ( fig. 3 ) runs through its centre. This axis is synonymous with the scale for Lightness. The non-chromatic axis starts with 0 at the bottom showing black, followed by continuously lighter greys ending with 100 on top.
The term of Chroma is being illustrated in figures 2 and 3 . The Chroma of a colour is the intensity of its colouring. It increases gradually starting at the central non-chromatic axis and ending outside. Its value on the axis is zero.
The structure of the RAL DESIGN System is not arbitrary. It follows an internationally used colour measurement system laid down by CIE (Commission International d°šEclairage) in 1976. The colour distances between the individual colours are defined by the CIELAB-colour distance formula. They are also embedded in DIN 6174.