They come in a variety of colors, such as green, yellow, purple, blue, red, maroon, etc.
Balls first appeared in Don't Lose Your Marbles, where they were only red balls and one maroon ball. The Announcer used the red balls in the episode's challenge, where they were scattered across an area for the contestants to find and return. Rocky was also an apparent source of red balls, which allowed both him and Tennis Ball to do the challenge. However, if a contestant were to return the maroon ball, points would be taken off.
They appeared again in This Episode Is About Basketball for the challenge which was basketball, where the objective was to throw the ball to the basket to weigh it down so that it can fall into the buzzer and be safe.
In the same episode, balls were used for more than weighing down the basket (unintentionally.) A ball smashed Cloudy's window (in Cloudy's Collection.) Black Hole also sucked up some of the balls. The balls also broke one of the baskets.