Conway pointed to bananas as an example of this phenomenon. Bananas can be a number of colors—green when they’re not yet ripe, brown when we’ve let them sit out for too long. But we label bananas yellow, and we do it because “this is their state when we care about them,” Conway said. “It is canonical among most people to call bananas yellow.”

By this logic, some people have internalized what they’ve seen at major tournament events and on labels from tennis manufacturers and come to associate tennis balls with the color yellow. Sure, we understand that tennis balls could be made in other colors, but the state of tennis balls when we care about them—such as when we watch a match on TV—is yellow, so when we’re asked to ascribe one color to them all, we go with that. As for why we haven’t agreed on the color of tennis balls as we have for bananas, “maybe they haven’t been around long enough, or the color of them has actually changed,” Conway said. This makes sense. Bananas have been around far longer than tennis balls.

The tennis gods picked yellow for the color of tennis balls because they thought yellow was bright enough for people to see it with ease. And that’s true, but just because something is highly visible to the eye doesn’t mean it’s easy for us to describe it. Red, green, blue, and yellow are “unique hues,” colors that human vision perceives as pure, rather than a mix of two or more. Among these hues, “yellow is the most precisely identified across people,” Conway said. “If you ask people to pick out ‘yellow’ in the spectrum (a color that is neither red nor green), pretty much everyone identifies the same wavelength.”

This shows that people can easily distinguish yellow from other colors. But how that yellow should be described is another question. “Yellow presents an interesting paradox: It is easy to discriminate, but we don’t name it as well as we name other colors like red and orange,” Conway said.

In other words, humans are good at pointing at a yellow paint chip in a line of colorful chips and saying, that’s yellow. But if we’re shown a yellow paint chip alone and asked what color it is, we become less certain about calling it yellow. In a recent study Conway coauthored that surveyed people who speak three different languages—American English, Bolivian Spanish, and an Amazonian language called Tsimane—researchers found that “language systems of people in cultures with little exposure to industrialization are pretty poor at communicating yellow.”

And what about green? “We are generally really bad, across all cultures, in communicating green,” Conway said.

Great.

“I can think of many more orange or red objects—apples, tomatoes, cherries, most fruit, faces, etc. So it might not be surprising that there is some disagreement about the color of tennis balls,” he said. “They are an odd color, designed to be odd so that they are especially visible on the court. But because they are odd, we haven’t resolved how to label them.”