The CIE *XYZ color space* [1] defines all colors in terms of three imaginary primaries *X*, *Y*, and *Z* based on the human visual system that possess the following properties:

- Based on experimental data of human color matching;
*X*,*Y*, and*Z*work like additive primaries RGB (every color is expressed as a mixture of*X*,*Y*, and*Z*);- One of the three values –
*Y*– represents*luminance*, i.e. the overall brightness (lightness) of the color as a function of wavelength; - All color values (
*X*and*Z*) are positive.

The *xyY color space* is derived directly from *XYZ* and is used to graph colors in two dimensions independent of lightness. The value *Y* is identical to the tristimulus value *Y* (in *XYZ*). The *x* and *y* values are called the *chromaticity coordinates* of the color and are computed directly from the tristimulus values *XYZ*. The chromaticity of a color was then specified by the two derived parameters x and y, two of the three normalized values which are functions of all three tristimulus values *X*, *Y*, and *Z* [2]:

The *xyY* values of colors can be plotted in a useful graph known as the *CIE chromaticity diagram*. Colors on the periphery of the diagram are saturated; colors become progressively desaturated and tend towards white somewhere in the middle of the plot. The point at *x* = *y* = *z* = 0.333 represents the white perceived from an equal-energy flat spectrum of radiation.

For any device such as a monitor or printer, we can plot its *gamut* – the colors that are reproducible using the device’s primaries [3]. This is one of the most common uses for the CIE diagram. The brighter triangle in the center of the plot shows the colors which can be reproduced by a standard computer screen, representing its gamut. Colors outside the triangle are said to be *out-of-gamut* for, and cannot be reproduced on, normal display screens. They are artificially desaturated in the plot.

For all its simple derivation from eye-response functions, the 1931 CIE chromaticity diagram is not *perceptually uniform* because the mathematics of *xyY* do not model distances between colors. This is the reason why the area of green-turquoise in the large lobe at the top left of the diagram (inaccessible to monitors) appears to be disproportionally large compared with the others. To solve this problem, other color-space coordinate systems and plots were invented – CIELUV [4], CIELAB [5] – in an attempt to be more perceptually-uniform. Their use is fairly specialized.

References:

*CIE 1931 color space*on Wikipedia- Danny Pascale —
*A Review of RGB Color Spaces … from xyY to R’G’B’* *Color Gamut*on Wikipedia*CIELUV color space*on Wikipedia*CIELAB color space*on Wikipedia