First, it was “The Dress.” Now, the internet is divided about the true color of tennis balls.

The debate started last month after YouTube creator CGP Grey tweeted a poll asking if people would describe the color of tennis balls as green, yellow, or something else (some have suggested they’re chartreuse).

Roger Federer offered his opinion Monday while in Chicago to promote the Laver Cup.

His answer did little to settle the debate, though.

Bevil Conway, a researcher who studies color perception at the National Institutes of Health’s National Eye Institute in Maryland, offered the following explanation when contacted by The Atlantic’s Marina Koren:

“I’m not looking at any tennis balls now, but I think they are yellow,” he said. “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.”