I remember this being discussed a couple times on here before by actual ball reps, fairly recently in fact. It's probably buried in the company-specific forums if you're willing to try a search. But I can say with about 99% certainty the ball companies came to this conclusion from experience and basic market research. Brightly colored balls don't really sell particularly well in the USA outside of a few exceptions from what I've seen (the Toxic for example was popular around here simply because it was so eye-catching).