There was the problem that colours used to be determined by a reference system, and all colours corresponded to that reference model.
But the ravages of time gnaw on such reference models, and a new system - one that doesn't require such colour models - was developed.
The "Commision International de L'Eclairage" (CIE) was assigned to develop such a system. Initial results were presented in 1931: A colorimetric model, the so-called CIE chromaticity diagram.
Today, this system is one of the most commonly used colour spaces.
The CIE colour system is based on mathematical calculations and a "standard observer" was defined through a series of tests. This standard observer conformed with the average human colour sensation.
CIE only requires two values - x and y - which can be calculated from the tristimulus values X, Y and Z (instead of R, G, B)
The CIE diagram is the shape of the sole of a show, or of a tongue. The spectral colours are located at the border of the surface. The white point (equal energy point) corresponds to coordinates x=0.33 and y=0.33.