A tennis ball argument

At physical therapy, Noam Hassenfeld heard people argue fiercely about whether tennis balls are yellow or green. Everyone insisted their perception was obviously correct and could not believe others saw it differently.

Meet Bevil Conway

Hassenfeld called neuroscientist Bevil Conway, who heads a lab at the National Eye Institute and is writing a book called Color Coded. Conway is also a working artist, which is how he got into studying color.

There's no such thing as color

Conway told Hassenfeld there is a good reason people disagree about tennis balls. His explanation: there is no such thing as a color. Color is not a property of objects but something the brain creates.

“There's no such thing as a color.”

Color takes brain power

Conway's lab discovered that the brain uses as much processing power for color as it does for face recognition. This means color involves high-level reasoning, social cognition, and visual memory, not just a simple filter.

“there's as much brain involved in processing color as there is in face recognition.”

Wavelength is not color

The classic definition says color comes from wavelength properties of light. But if that were true, a peach under blue light would look blue and under yellow light would look yellow. Instead, it always looks like a peach.

Color constancy

A peach looks the same color indoors and outdoors even though the wavelengths coming off it are very different. This is color constancy: an object's color stays constant despite viewing conditions, which wavelength alone cannot explain.

The brain is not deceiving you

Conway says the mistake is thinking color is wavelength. The peach is not fundamentally a different color inside and outside. Color is not wavelength. It is dependent on your visual system interacting with the world.

“Color is not wavelength!”

Color is made by your brain

Color is a generative product of the brain that gives reliable, useful, true, meaningful information about the world. But color itself does not exist out there independent of a viewer. Your brain creates it.

“Color is a generative product of the brain”

The viral dress explained

Ten years ago, a photo of a dress went viral. Some people saw blue and black, others saw white and gold. Conway says this shows color is generative and constructive, with the brain making assumptions about lighting.

How the dress works

The dress photo is blurry with little context. If your brain assumes warm, direct sunlight, it subtracts yellow and you see blue and black. If you assume cool, shaded light, it subtracts blue and you see white and gold.

Why people got upset

The dress was the most viral image in internet history. People felt unnerved because they could not believe others saw it differently. Society depends on shared experience, and the dress seemed to threaten that shared reality.

“Do we live in a shared reality?”

Green and blue are vague

When asked to pick their best example of green, blue, red, orange, or yellow, people show the most variability with green and blue. One person's perfect green is another person's perfect blue. This is true across all cultures.

Why green and blue are different

Green and blue are not tied to objects the way other colors are. Most natural objects are yellow, brown, red, pink, or purple. Green is the forest and blue is the sky, the background. There are very few natural blue objects.

Yellow is consistent

When people in the rainbow pick where yellow is, they are incredibly consistent. People really know yellow. This is different from green and blue, which people perceive with much more variability.

“They really know yellow.”

Tennis balls are fluorescent

Tennis balls are a little bit fluorescent, emitting more light than they take in by reprocessing short-wavelength light. They are bright yellow, right at the peak of human visual system sensitivity, which is technically closer to green.

Why people see yellow

The tennis ball is at the optimal wavelength humans can detect. Because it is so bright and yellow is considered the brightest color, people are persuaded to see it in a yellow direction, even though it is technically closer to green.

Both answers are right

Conway agrees the tennis ball is technically closer to green than yellow. But he still likes it as yellow. The ball is genuinely both, depending on how each brain processes the same wavelengths of light.

“I still like it as yellow.”