All colours that are judged as ‘unique red’,  appear ‘neither yellowish nor bluish’. That means the yellow-blue mechanism (YB) is silenced and its output is therefore zero: 

α*L + β*M +  ψ*S = YB = 0  .

All reddish colours that satisfy this equation are ‘unique red’ and will lie on a plane in LMS cone space (and any linear transformation).  Similar equations can be derived for unique green, yellow and blue.

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