The three tristimulus values, XYZ, form a color coordinate system.|
In order to represent individual colors in a descriptive manner, the standardized red and green color fractions x and y are projected into a planar coordinate system (often called "shoe sole" for its shape).
|This xy representation is independent of the brightness of the coloring and produces all object colors that can be produced. Moreover, it allows the wavelengths of the colors triggering a monochromatic color stimulus to be determined. The so-called line of purples, which represents the violet hues between red and blue, is situated in the lower region.|
However, one deficit of the xy chromaticity values is that color distances in this coordinate system do not correspond to differences in color as they are perceived physiologically.