…Of nearly 30,000 participants, 52% said a tennis ball is green, 42% said it’s yellow, and 6% went with “other.” I was stunned.

…According to the ITF, tennis balls were once actually white or black. The arrival of television changed that. Viewers had trouble seeing tennis balls as they hurled across the court in televised matches, so the ITF instructed tournaments to start using yellow ones in 1972 (though white ones were still allowed)…Only Gamma Sports took my query seriously and responded. They put their answer in all caps: “OPTIC YELLOW!” (My colleagues who played tennis or had worked at tennis clubs—the ones with the most experience in actually looking at tennis balls—also thought they were yellow.)

…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 3 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.

…The discussion over tennis balls began to resemble another color-related debate: the question of The Dress…Back in 2015, Conway and other experts explained that the difference of opinion about The Dress stemmed from the way the human brain evolved to perceive light (they’ve since fleshed out the theory in a recent paper). We experience all kinds of warm and cool light throughout the day. We get warm light from sunsets and incandescent bulbs, and blue light from overcast skies and computer screens, to name a few…If the same effect is true for our perception of tennis balls, then the people who see the dress as white and gold, because they are predisposed to discounting cool colors, should see the ball as yellow. Meanwhile, those who see the dress and blue and black, because they discount warm colors, should see the ball as green.

And that’s exactly the effect we found, according to a quick, very informal survey of my Slack team. Aside from one or two outliers, those who believe a tennis ball is yellow saw the dress as gold and white, while those who believe a tennis ball is green saw the dress as black and blue. Minds blown.

Conway took it a step further, suggesting that the way people see tennis balls could reveal something about their lifestyles. Night owls, for example, spend most of their time under artificial, warm light, which means they’d discount warm colors and see a tennis ball as green. Early birds, on the other hand, get plenty of exposure to blue daylight, which means they would discount cool colors and see a tennis ball as yellow. “I’d emphasize that this is just a theory, and we’d need lots of data to support it before I’d believe it were true”, Conway said.

…My colleague Julie Beck summed up the ordeal with a sentiment we could all agree with. “It is truly horrifying every time it gets pointed out that we’re all walking around thinking we share the same reality”, she said. “And we just are not.”

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