I think the research you saw is baloney. My Jack Russel, before he went blind, would only fetch his red ball. He had 3 balls all exactly alike, those little ones with feet you get at the pet store. He would only fetch the red one.
I had a Lab who knew his toys by color, had several Kongs, blue, red and yellow. He’d get the right one when asked. If they’re “colorblind” they’re able to distinguish via some other means, scent, taste.
Given the dog’s sense of smell, there could other reasons why a red ball was his favorite. My dog loves red apples and hates green ones. It is the sweetness vs. the tartness and he can smell that in an intact apple. OTOH, his favorite toys are either green or blue and he has a bias towards anything that rolls. The toys that retain scent are always his choice when it comes to settling in for a good chew.
The research was anatomically based. They charted cones and rods in the retina. It was all done with imaging, discounting the dog’s reactions to specific items. I tend to accept it.