Chromaticity is an objective specification of the quality of a colour regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colourfulness (s). The number of parameters is due to the trichromatic nature of human colour vision.

Colourfulness is alternatively called saturation, chroma, intensity or Excitation Purity.

Simply put, while colour spaces are 3D, chromaticity is their 2D representation.

Quantitatively, the the white point of an illuminant or a display is a neutral reference characterized by chromaticity, and all other chromaticities may be defined in relation to this reference using polar coordinates. The hue is the angular component and purity is the radial component.

Coordinate Systems

xy for xyY colour spaces, which are normalized xyz spaces of CIEXYZ. The luminance Y is preserved and augmented.
rg chromaticity space - two dimensions of normalized RGB, or rgb space. Mostly used in computer vision.

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