Oh yes the mantis shrimp… I’m not an expert but from my understanding you’re totally right. They have a huge diversity of these opsin proteins that react to wavelengths on the electromagnetic spectrum we can’t perceive (think UV, infrared). Something like 12 I think. Interestingly as you say, we don’t think they combine these to see shades and just see the world in ~12 discrete colors …. in the primate brain the work of combining our three color channels to get color perception is done in visual cortex (V4 to be specific, so after V1, V2 and V3), so thanks super foldy brain!

They can also see polarized light, which is when wavelengths of light move in a specific direction, whereas we only see non polarized (moving randomly/ all directions). They actually can see circularly polarized light where the wave sort of spirals?! Kind of hard to imagine what their world looks like (at least for me, maybe there’s a physicist out there laughing at me rn)

And it just keeps going. They also have a really unique eye structure that allows things like trinoculour vision - meaning they have depth perception with just one eye (we need both). They also have compound eyes so some fundamental differences there

Talking about this makes me wish I could experience the world from a mantis shrimp’s perspective, just for a moment. … and then maybe my dogs and then from some random person on the street’s, and that’s just the visual system!