This would seem to explain why when you go to the optician, he's got a lot of red/green tests but never blue one as far as I can tell. The critical graph is the

This would seem to explain why when you go to the optician, he's got a lot of red/green tests but never blue one as far as I can tell. The critical graph is the one with the blue peak to the left and the red and green near each other on the right. Also it seems to hint that there's a fourth receptor that humans don't have in the gap region. Tetra-chromatic creatures do exist IIRC.