What colour is a tennis ball?

It’s the dress part two!

I make this decision as much on the basis of what I think I know about tennis balls—that they are yellow—as I do on what color I recall that they looked when I last saw one… In other words, like the color of a lot of objects, how we label [a tennis ball] is determined both by perceptual and cognitive factors: the actual physical light entering your eye and … knowledge about what people have typically labeled the objects.

I have to say, it never occurred to me that a tennis ball might be any colour other than yellow.