The perception of color is completely beside the point. The point of a universal is that it exists, even if it cannot be perceived at all.
Would you care to shift universals with RWP and try pi or threeness?
For the record: you shifted universals. I took that as a concession that you're no longer claiming green is a universal. If not, let me know, and we can discuss how loose the criteria for a universal can be.
On a related note (with practical applications), one can ask if there is a "Platonic" e-flat (or any other tone.) Also, is the a "Platonic" major third or tritone?