CIELAB

The CIELAB Colour space, also referred to as $L^{\ast }{a}^{\ast }{b}^{\ast }$ is a Colour Space defined by the International Commission on Illumination as one of the CIE 1976 Uniform Colour Spaces, alongside CIELUV.

It expresses colour as three values: $L^{\ast }$ - Perceptual lightness $a^{\ast }$ and $b^{\ast }$ - the four Unique Colours of human vision: red, green, blue and yellow.

CIELAB, along with CIELUV produces colours that are more perceptually linear than other colour spaces.

It is defined with respect to a White Point, for which CIE recommends the D65 Standard Illuminant.

The $a^{\ast }$ and $b^{\ast }$ values are unbounded, and can easily exceed $\pm$ 150 to cover the entire colour gamut. But software implementations often trim the values. For example, in integer math, a range of -128 to 127 is chosen.