When I started to watch Ace wo Nerae! I was intrigued by the color of the balls. A little research led me to the International Tennis Federation webpage.
From the link: Historically, balls were either black or white in colour, depending on the background colour of the courts. In 1972 the ITF introduced yellow tennis balls into the rules of tennis, as research had shown these balls to be more visible to television viewers. Meanwhile Wimbledon continued to use the traditional white ball, but eventually adopted yellow balls in 1986.
