Rather than asking people for colour names or colour categories, we ask observers to select the colour which appears ‘neither red nor green’ (yielding unique yellow and unique blue) or ‘neither yellow nor blue’ (yielding unique red and unique green).

The figure above shows an example of a typical stimulus to asses ‘unique red’. The observer selected that patch which appeared neither yellow nor blue.  Since we obtain these unique hue settings at different luminance (V) and saturation levels (S),  as shown in the figure below, we have many points  in 3-dimensional (LMS) cone space  that correspond to a particular hue. This will allow us to determine quantitatively how the cone signals are related to a particular perceived hue.

Hering and Urfarben

The idea of ‘unique hues’ was first mentioned by Ewald H Hering (1834-1918), who referred to these colours as ‘Urfarben’, see figure below. Colour-normal human observers have usually no difficulty in making these hue judgements.

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