Interconnected

A tetrachromat is a person who distinguishes four primary colours, not three. There have been reports of tetrachromats, mostly women. But there's a claim that the human is a blocked tetrachromat -- that is we all have the physical ability in the eye to distinguish four independent colours, but the wavelengths of one of them are absorbed by the physical stuff in the eye before it gets to the cones at the back [via Captain Cursor].