systems of the  
  twentieth century  
CIE L*a*b colour  

Neither RGB or HSV represent all possible hues on the CIE chromaticity diagram. CIE L*a*b colour attempts to do this, while compensating for the distortion of colour on the original chromaticity diagram.

'L' represents luminance or lightness, while 'a' represents green to red and 'b' represents the blue to yellow axis.

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A sphere with a vertical axis (L) running from white down to black. Red, blue, green and yellow run around the equator and blend into each other. A horizontal axis (a) connects red to green and another (b) blue to yellow