From psychophysical, physiological and anatomical investigations, we know that cones in the normal human retina contain one of three types of photopigment. An individual with normal color vision is therefore called a trichromat. From the discussion above it follows that a trichromat requires a mixture of three wavelengths to match any other wavelength.
The ability to match colors using a mixture of just three primaries forms the basis of a powerful method for describing colors. In 1931, the Committee Internationale de l'Eclairage (CIE) devised a standard system for specifying colors based on the trichromatic color matching characteristics of a 'standard observer'. For technical reasons the CIE adopted 'imaginary' primary colors (rather than red, green and blue) which were designated X, Y and Z. The relative amounts of X, Y and Z required in a color match are represented by the lower case letters x, y and z. Since the sum of x+y+z is equal to unity, the value of z can be ascertained from the values of x and y. Therefore, all perceptible colors of a given brightness can be represented on an xy graph. A representation of the CIE color space can be viewed from the View menu of the computer program
Spectral colors are arranged along the arc of the perimeter of the diagram. White, being an approximately equal mixture of the three primaries, falls in the centre of the diagram.