Originally, tennis balls were not yellow. They were white, even black, because of their wool composition. But towards the end of the 1970s, colour television made its appearance. Tennis is more and more broadcasted but the white balls are disturbing because they merge with the white lines of the court.
In 1972, in order to offer better visibility to viewers, broadcasters and the International Tennis Federation decided to introduce yellow felt balls into the circuit. But not just any yellow. A fluorescent yellow called "optic yellow" determined by research to be the most visible colour on TV. Since then, all tournaments have been using this standard. But, officially, white balls are still allowed on the professional circuit and encouraged on cool courts.