As far as I'm aware the color of the iris has little or nothing to do with what happens to the pupil and/or retina, which are the parts that deal with how much light is getting into the eye (pupil) and impacting the light-sensitive part of the eye (retina). Perhaps infinitessimaly less light is transmitted through the pupil of a dark eye since the dark iris might absorb a bit more than a light one (this is the theory behind football players putting dark streaks under their eyes) but I doubt it's a significant difference.
Still it's a thoughtful idea if the horse seems sensitive or uncomfortable when out in the snow. I know I like to wear dark goggles or glasses on the rare sunny days here when there is snow on the ground!
Lighter eyes can sometimes have trouble knocking down the glare of light. But heck, snow blindness is a real issue for dark eyes as well.
I don't usually have to, but on the few occasions we've had solid snow cover for more than 1-2 days and the sun shines, I do find my horses' eyes start to water, so I do put a fly mask on for the days.
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET
A friend of mine has a cremello paint with 2 blue eyes along with the pink eyelids, the horse always has a flymask one whenever the sun is shining, snow or summer. Poor guy squints and his eyes water if someone forgets or the weather changes during the day.