All colours that are judged as ‘unique red’ silence a  yellow-blue (YB)  mechanism and lie on a plane in LMS cone space. The normal vector for the red null plane defines the chromatic properties of this YB mechanism.  This figure below shows that the mechanism that is silenced by red is almost exclusively a mechanism driven by S cones (and almost parallel to the L,M plane) with a very small (opponent) L-M cone input. Hence, when human observers perceive something as ‘uniquely red’ the S cones don’t respond.

Similar null planes can be derived for unique green, blue and yellow.

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