Squash is a ball sport played by two (singles) or four players (doubles) in a four-walled court with a small, hollow rubber ball. The players must alternate in striking the ball with their racket and hit the ball onto the playable surfaces of the four walls of the court.

Squash balls are between 39.5 and 40.5 mm in diameter, and have a weight of 23 to 25 grams. They are made with two pieces of rubber compound, glued together to form a hollow sphere and buffed to a matte finish. Different balls are provided for varying temperature and atmospheric conditions and standards of play: more experienced players use slow balls that have less bounce than those used by less experienced players (slower balls tend to "die" in court corners, rather than "standing up" to allow easier shots). Small colored dots on the ball indicate its dynamic level (bounciness), and thus the standard of play for which it is suited.

The recognized speed colors indicating the degree of dynamism are:

  • Color Speed (of Play) Bounce Player Level
  • Double yellow Extra Slow Very low Experienced
  • Yellow Slow Low Advanced
  • Red Medium High Medium
  • Blue Fast Very high Beginner/Junior

More Info: en.wikipedia.org