The video description has the answer: "Out black lab LOVES tennis balls. His dream came true when a local tennis instructor gave us 300 used balls." Three hundred!

But, according to the book 2,024 QI Facts To Stop You In Your Tracks, at some point, David Attenborough noticed that the ...

Those used initially were black or white, depending on the background color of the courts, according to the International Tennis Federation (ITF). The ITF then changed the color of the balls in 1972.

For nearly a century, tennis balls were white or black. It wasn’t until 1972 that tennis balls took on their bright neon hue. At the time, Attenborough was working as a studio controller for the ...

Thus, until the 1970s and for almost a century, tennis balls were white or black. Today they are yellow, although some recalcitrant still speak of 'green'.