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HSL and HSV (Hue Saturation and Lightness / Value)

Other similar colour spaces include HSI (intensity), HCI (chroma / colourfulness), HVC, TSD (hue saturation and darkness) etc etc

As linear transforms from RGB they are device dependent, non-linear but very intuitive. The separation of the luminance component has some advantages in image processing. There appears to be little point in using these associated colour spaces, a better alternative is CIELch, which although more complex offers a more accurate, but equally intuitive way of specifying colours.

If you think of RGB space as a simple cube, with the red, green and blue vectors as the three axis and imagine a line drawn through the cube centre from black (0,0,0) to white (1,1,1) then HSI space is this cube tipped up onto one corner so that the black - white line is vertical. Then the black - white line becomes the intensity axis, the hue is given by an angle from the intensity axis and the saturation is the distance from the central intensity axis out to the colour point (the radius). If you only want a simple way of selecting colour on an RGB display the following transforms can be useful:

HSI: Hue Saturation Intensity.
HSV: Hue Saturation Value.
HSI: Hue Saturation Lightness.