Originally hollow celluloid, the balls became solid in the plastic era with a white version joining the range with the advent of the Floodlighting edition.
The card set ball (far left above) is very light and slightly larger than its successors. With the switch to the more robust celluloid figures the ball got slightly smaller and thicker / heavier (middle left).
The white ball apparently started life as the pea in the football range whistle. Perhaps because its easier to see it feels like its slightly easier to bat against than the standard red ball. The white balls are a little harder to find if you’re looking to buy, but are by no means rare.
Most box sets provided two balls, but when sold in separate packs they were in fours.