The document discusses the chromaticity diagram, which is a plot of y versus x chromaticity coordinates that represents all possible colors. It can be used to determine properties of colors like dominant wavelength, excitation purity, and whether they will appear neutral, saturated, or as shades of spectrum colors. However, the chromaticity diagram is two-dimensional and does not fully represent color, with the third dimension usually taken to be the Y tristimulus value, which indicates lightness.