http://www.10andundertennis.com/parents/about/equipment-for-kids/
What is 10 and Under Tennis?
10 and Under Tennis follows the same logic as other youth sports like baseball or soccer, which use kid-sized courts and kid-sized equipment. Kids learn to play baseball by first playing T-ball; they use shorter, lighter bats and larger, softer balls. Kids learning basketball shoot baskets with kid-sized balls on lowered backboards and they play soccer on smaller fields with smaller goals. Now with 10 and Under Tennis, balls bounce lower, don’t move as fast through the air and are easier to hit. Racquets are sized for small hands and courts are smaller and easier to cover. By using this format, the benefits are immediate and within a short time kids are rallying, playing, and excited to keep playing. That means kids will have more fun and less frustration. They’re playing real tennis and having real fun—and that’s what is most important.
Red foam and red felt:
For kids 8 years old and under, a red ball moves slower through the air, bounces lower and travels less distance.
Orange ball:
For kids 9 & 10 years old, an orange low-compression, ball moves a little faster and travels farther than the ball used with the younger group but still has a lower bounce than the standard yellow ball.
Green ball:
For kids ages 10-12 years old. As children become more accomplished with their strokes and tactics, they can move to the full-sized court and use the green ball, which is slightly lighter and slower-moving than a yellow ball, giving children more time to get to and return shots.
Thanks maggie - I could have googled but was lazy!