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.
I still am in doubt because when I threw him a green ball or a yellow one, he would start after it and then turn around and refuse to bring it back to me. He didn’t get close enough to smell or taste it. Now that my little guy is totally blind, he doesn’t play ball anymore but his sense of smell and hearing have improved tremendously.