The CIE colorimetric system is a method of numerically specifying the physical attributes of colours and is primarily of use in the instrumental measurement of colour. The basis of the system is that a colour stimulus can be matched by an additive mixture of three primary colour stimuli of variable proportions. A primary colour stimulus is a colour which cannot be colour matched by a mixture of the other primary colour stimuli. The CIE colorimetric system is only incidentally associated with collections of physical samples, and as such no CIE atlas exists. Additionally, as the CIE colorimetric system is not based on perceptual qualities of colour any such atlas would be of little use to designers or architects. The CIE system is a method of specifying colours solely by their physical attributes, and by general consensus the CIE colorimetric system, CIE standard observers and CIE standard illuminants are the global standards of specification. Lightness forms the central axis of the system with the x and y axes perpendicular to the lightness axis and each other. Planes of constant lightness form xy chromaticity planes.
The x axis approximates to the green/blue to red/orange axis and varies in extent based on Lightness.
The y axis approximates to the purple to green/yellow axis and varies in extent based on Lightness.