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.
It could simply be he likes the SHADE he sees with the red. The red probably looks different from the other colors, a gray perhaps, and he likes that better.
Or, when did you buy them? What order? Same store?