The CIE has defined a system that classifies colour according to the HVS (the human visual system). Using this system we can specify any colour in terms of its CIE coordinates.
The CIE system works by weighting the SPD of an object in terms of three colour matching functions. These functions are the sensitivities of a standard observer to light of different wavelengths. The weighting is performed over the visual spectrum, from around 360nm to 830nm in set intervals. However, the illuminant, and lighting and viewing geometry are carefully defined, since these all affect the appearance of a particular colour. This process produces three CIE tri-stimulus values, XYZ, which are the building blocks from which many colour measurements are made.