In 1931, the International Commission on Illumination (CIE) published the CIE 1931 color spaces which define the relationship between the visible spectrum and human color vision. The CIE color spaces are mathematical models that comprise a "standard observer", which is a static idealization of the color vision of a normal human. A useful application of the CIEXYZ colorspace is that a mixture of two colors in some proportion lies on the straight line between those two colors. One disadvantage is that it is not perceptually uniform. This disadvantage is remedied in subsequent color models such as CIELUV and CIELAB, but these and modern color models still use the CIE 1931 color spaces as a foundation.

The CIE developed and maintains many of the standards in use today relating to colorimetry. The CIE color spaces were created using data from a series of experiments, where human test subjects adjusted red, green, and blue primary colors to find a visual match to a second, pure color. The original experiments were conducted in the mid-1920s by William David Wright using ten observers and John Guild using seven observers. The experimental results were combined, creating the CIE RGB color space. The CIE XYZ color space was derived from CIE RGB in an effort to simplify the math.

These color spaces are fundamental tools for measuring color for industry, including inks, dyes, and paints, illumination, color imaging, etc. The CIE color spaces contributed to the development of color television, the creation of instruments for maintaining consistent color in manufacturing processes, and other methods of color management.

Normal human color vision is trichromatic, which is enabled by three classes of cone cells (L, M & S). Each cone class contains a slightly different photopsin with a different spectral sensitivity. The spectral sensitivities are summarized by their peak wavelengths, which are at long ("L", 560 nm), medium ("M", 530 nm), and short ("S", 420 nm) wavelengths, sometimes shorthanded inexactly as red, green and blue cones, respectively. The differential excitation levels of these three cones comprise the tristimulus values, denoted "L", "M", and "S", and are the parameters that define the 3-dimensional "LMS color space", which is one of many color spaces devised to quantify human color vision. In principle, any human color sensation can be described by a set of tristimulus values. A continuous spectral power distribution of light ${\displaystyle S(\lambda )}$ is converted to the discrete tristimulus values (in this case ${\displaystyle L}$, ${\displaystyle M}$ & ${\displaystyle S}$) by integrating over a spectral sensitivity of one of the cone classes (${\displaystyle {\overline {L}}(\lambda )}$, ${\displaystyle {\overline {M}}(\lambda )}$, or ${\displaystyle {\overline {S}}(\lambda )}$):

These are all inner products and can be thought of as a projection of an infinite-dimensional spectrum to a three-dimensional color. This LMS color model is refined to the LMS color space when the spectral sensitivity "primaries" are defined according to the standard observer. The LMS color space can be further transformed into similar three-dimensional color spaces, such as RGB, XYZ, HSV or cognates thereof. The tristimulus values associated with a color space can be conceptualized as amounts of three primary colors in a trichromatic, additive color model. In some color spaces, including the LMS and XYZ spaces, the primary colors used are not real colors in the sense that they cannot be generated in any spectral power distribution of light.

Since a lot of information is lost during the conversion (projection) of a continuous light spectrum to tristimulus values, it follows that there are disparate spectra that can stimulate the same tristimulus values. These disparate spectra are known as metamers. For example, a monochromatic 570 nm (yellow) light is metameric with a dichromatic light spectrum comprising 2 parts monochromatic 535 nm (green) light and 1 part monochromatic 700 nm (red) light (accounting for luminosity). In 1853, Hermann Grassmann developed Grassmann's laws, which aimed to describe color mixing algebraically. These laws laid the theoretical framework necessary for color experiments performed by Hermann von Helmholtz (remembered for popularizing the trichromatic theory) and James Clerk Maxwell in the 1850's, and later in the experiments used to develop the CIE 1931 color spaces. The laws can be summarized in three principles:

These laws assume that human color vision is linear, which is approximately true, but non-linear effects (such as the Helmholtz–Kohlrausch effect) are not considered in the CIE 1931 color model.

The CIE 1931 color spaces are 4 interrelated color spaces with the same origin. In the 1920s, two independent experiments on human color perception were conducted by W. David Wright with ten observers, and John Guild with seven observers. How their results laid the foundation of the CIE 1931 color spaces is described in this section.