Is a tennis ball green or yellow?

Well, actually, they aren't. The International Tennis Federation (ITF) categorizes the color as "optic yellow.” Incidentally, tennis balls actually used to be black or white. When tennis started to be televised in color in the late 1960s, it was hard for viewers to track the ball on their screens, so the ITF came up with the bright greenish-yellow to make it easier on their eyes.

They're rare today, but according to ITF rules, white tennis balls are technically still allowed—a relic of pre-TV times.