Did you know ? Tennis balls haven’t always been yellow. At first they were white, like on the picture.
Unfortunately, when television arrived, it was a little complicated to follow the ball on black and white screens. They found out that yellow was easier to see than white on those old screens. Indeed, the contrast between the ball and the ground was higher on every type of court (grass, clay or hardcourts of concrete).
Therefore, in 1972, the International Tennis Federation made it official that tennis balls will be white or yellow. Indeed, you can still play with white ones in official competitions even if yellow ones became the standard.
By the way, if you want to know more about the strict specifications of a tennis ball, it has to have a diameter between 6.35 and 6.67 cm, a weight between 56.70 and 58.45 g and it has to bounce higher than 1.34 m but lower than 1.47 m when dropped at 2.50 m above the ground.