[GrantMoyer]

Humans perceive all stimulation in the same raito of the L, M, and S cones to be the same color, but with different brightnesses. So only two dimensions are nessesary to represent human visible colors, hence HSV or L*a*b* space.

[dahart]

There is a fair point there, but a few things - HSV and Lab are only models, they don’t necessarily capture all visible colors (esp. when it comes to tetrachromats). Brightness is a dimension, and can affect the perception of a color, esp. as you get very bright - HSV and Lab are 3D spaces. Arguing that brightness should be ignored or factored out is problematic and only a small step from arguing that saturation should be factored out too and that color is mostly one dimensional.

[a-priori]

According the opponent process model of colour perception you need three axes to represent all colours: luminosity [L+M+S+rods], red-green [L-M] and blue-yellow [S - (L+M)].