The sky is blue due to pure physics and the refraction of the chemicals therein. Color is merely a spectrographic measure that is displayed in the visual spectra. Your argument doesnt hold water when you start bringing up things like spectra .
Flowers and angiosperm sex organs evolved in tandem with higher insect orders where color CAN be seen in the complex crystalline insect eye.
Some snakes DO have POS's and we infer that they see color. so the brightly hues xcrotalids and advanced elapids do have color schemes (look at a coral snale which is one qwith a POS).
Color, in order to be set in the genome (as we infer from fossils only) would have to serve some purpose, be it sexual display, camouflage, or intimidation. I wouldnt get too attached to the color thesis, its good but , when its proposed at some conference, a gazillion paleodudes and dudettes will tear him a new one . Its up to the proposer to back up his hypothesis with something besides cartoons.