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?
I bought the red one first and when it got lost for a while I bought the others. They were all exactly the same size and type except the color. I thought he would be happy to play ball again when I bought the green one, but no dice!