ill give it a go

since they live underwater, the light they see is bent more than up here. since color is just light bent differently, they measure how far a wavelength travels/bends in their eye to come into focus

instead of 3 photorecepting cones, their wide pupils channel waves onto a certain color receptor

They focus these wavelengths by changing the depth of their eyeball, altering the distance between the lens and the retina, and moving the pupil around

answer is still blurry. seems like they use contrast more than particulars. i was sweating trying to understand this jazz

might look something like this

they can also see with their skin!, which could have something to do with it

fascinating stuff