While there’s always some subjectivity in visual perception, some colors are supposed to be constant. Oranges are always orange. Fire trucks are always red. Tennis balls are always yellow.

Or maybe they’re always green.

In The Atlantic, Marina Koren explored the controversial debate over where the ubiquitous felt ball seen on courts or shooting out of serving machines falls on the color spectrum. After discovering that her colleagues perceived the ball different ways—either yellow or green—she attempted to understand why and if there was a definitive answer lurking in those ball cans.

By rule of tennis law—specifically, the International Tennis Federation, or ITF—a tennis ball should be yellow in color. The edict was handed down in 1972, after television viewers had trouble following the movement of white balls. Manufacturers like Gamma Sports also identify their product as yellow—in Gamma’s case, optic yellow.

So why do some people perceive the balls as green? For one thing, a yellow hue presented by itself can be hard for some people to describe. Yellow is easy to identify when contrasted with other colors—think paint swatches—but harder for people to articulate when there’s nothing to compare it to. Second, people tend to make color corrections based on lighting conditions. Some may discount warm colors like gold or cool colors like blue, changing how they perceive and interpret the color spectrum. If they discount cool colors, the ball might appear to be yellow. If they discount warm colors, green.

It’s possible that people who are active in the evenings under artificial light are more likely to discount warm colors, while people active in the daytime and under natural light would discard cool colors, further altering their perceptions.

Objectively, a tennis ball is yellow. But whether it appears that way to you depends on how you see the world.

[h/t The Atlantic]