Nowadays, every age/level has its own color, with the ball having a different bounce height and hardness depending on the color:

• Blue. To get acquainted with tennis in a playful way when you are 4 or 5 years old.
• Red. For young children. Played on a half court (only the so-called service boxes) with extra soft, red/yellow balls.
• Orange. One level higher. Played on a three-quarter court (an extra line between the baseline and the service box) with soft orange/yellow balls.
• Green. Another level up. The game is played on a full court with green balls. The yellow balls with a green dot are slightly softer than the normal yellow balls.
• Yellow. Tennis as played by seniors with the hard yellow balls.

Check the  Tenniskids website for detailed information!