A tennis ball is a small, hollow ball used in professional and recreational games of tennis. Tennis balls are fluorescent yellow in organized competitions, but in recreational play, other colors are also used. Until 1972, tennis balls were white (or sometimes black). In 1972, the International Tennis Federation introduced yellow balls, as these were easier to see on television, and these quickly became generally popular. Wimbledon continued using white balls until 1986.

Source: Wikipedia