Psychologist Alice Skelton has spent a lot of time with babies, plopping them down in front of computer screens to try to understand how they perceive the world. Babies are an extremely cute but also frustrating study set. “Sometimes they fall asleep,” she laughed, “then you have to throw the data away.”
Her work is based on the basic principle that a baby will pay less attention to something familiar and more attention to something new. Show babies a series of pictures of cats, for example, and they will start to lose interest as more and more cats are presented. “They get used to it, and they look less and less,” explained Skelton, who works at the University of Sussex. Throw in a dog, and their attention is piqued again: they’ll spend more time looking at the novel picture. This strategy can be used to see if and how babies categorize things before they have words for them.
What Skelton was investigating was color. Do babies think that all kinds of blue belong together in one pot, or do they see cyan and navy as radically different beasts? Do babies draw a line between green and blue the same way many adults do? And how many pots of distinct color are there in a baby’s brain?
Skelton ran tests across the full range of the color spectrum with hundreds of four- to six-month-old babies. As she expected, she found that they did indeed categorize color into a small set of discrete pots. But the number, she says, was a surprise. They had five: red, yellow, green, blue, and purple.
To understand why Skelton was surprised, and why anyone would do this work in the first place, we need to think a bit more deeply about color. It turns out that the colors we humans see, think, and talk about are shaped by a huge range of things: the wavelength of the light reaching our eyes; the receptors designed to detect this light; our brain’s interpretation; our cultural understanding of the result; and the language we use to talk about it. There’s plenty of room for fuzziness and difference all along the way. “It’s complicated,” Skelton sighed.
Not everyone agrees with Skelton’s conclusion that babies have five pots of color. But there is broad consensus that babies are one of the best windows into the brain as it exists before it gets modified by years of culture, language, and experience—some would say they’re our only window. Peer into a baby’s brain, and you should be able to see whatever basic bits of perception are hardwired in. Ask an adult, by contrast, and you’ll get answers colored by the experiences that have shaped, re-formed, and re-wired our circuits.
Many people in the modern Western world would say that there are seven colors in the rainbow: red, orange, yellow, green, blue, indigo, and violet (often remembered as ROYGBIV). That’s what vision scientist Rhea Eskew from Northeastern University in Boston remembers teaching his own kids. Yet there are no lines in a real rainbow: it’s an uninterrupted stream of different wavelengths of light. The concept of a seven-color rainbow dates to Sir Isaac Newton, the seventeenth-century English mathematician most famous for his theory of gravity. He chose seven colors in part because the number seven was auspicious: there were seven musical notes in the major scale, and this had long been thought to be meaningful.
Many ideas about color have changed over time. The word “orange” didn’t arrive in the English language until the fruit was imported, and its use for color first appeared in the 1500s. Few people today would consider “indigo” a basic color, added Eskew. Neuroscientist Bevil Conway, who studies vision at the National Institutes of Health in Washington DC, said that for his 10-year-old twins, the blockbuster movie Frozen has made turquoise “super important.”
And ideas about color differ dramatically from place to place: some cultures have evolved with surprisingly few color words, while others draw the line between basic colors in surprising places.
A language may have hundreds of color words (like crimson, vermillion, scarlet, and maroon), but there’s usually broad cultural agreement on a smaller set of basic colors (like red). These are usually—but not always—words that have no other meaning, are short and simple, and are frequently used and agreed upon.
In 1969, two anthropologists published a summary of the basic color terms used in languages around the world. Over the years, they extended that into a 2009 monograph, collating information from missionaries working in 110 languages.
Their resulting World Color Survey concluded that all the surveyed cultures have a word for black (or dark/cool) and white (or light/warm). More color-complex languages add a term for red; then for yellow or green (first one alone, and then both categories); and then a distinct blue. Many languages, including that of the Himba of Namibia, stop at five: light, dark, red, yellow, and green/blue (a sweeping category often called grue). The most color-complex languages (including English), the survey found, have 11 basic terms: red, orange, yellow, green, blue, purple, pink, black, white, gray, and brown.
Although it might sound like the World Color Survey turned up a lot of diversity in color talk, it actually emphasized just how shockingly similar most cultures are in their treatment of hue. It proposed that there is some universal experience of color that winds up represented in language.
More than a decade later, the World Color Survey still stands as a monumental achievement in big data collection—and its conclusions are still controversial. One major issue is that it’s surprisingly difficult to get this data, and the results are subject to interpretation and cultural bias. Some cultures, for example the Indigenous Candoshi of Peru, don’t have the category word “color” in their language, so it is impossible to ask, “What color is this?” Instead, one has to ask, “What is this like?” If the answer is “it is like ripe fruit,” is that an expression of color, or not? Opinions differ and may depend on the cultural expectations of the person doing the study.
“There are a lot of problems with, basically, white people doing [this] science,” said Skelton. “I’m not sure if you did the study again today, you’d do it the same way.”
Different cultures divide up colors in ways that seem odd or counterintuitive to others. They may group hues together because they are perceived as gendered—or some other way of sorting the world. They may put more emphasis on saturation or brightness over hue. Maybe they have no good reason to sort things by color, said Conway, the same way that English speakers don’t typically sort things by smell (the Jahai of Malaysia, on the other hand, do). It’s really only in an industrial society with manufactured objects that you’ll find several things that are identical except for color (like cars or T-shirts), thus requiring a color word to distinguish between them.
But if the World Color Survey is right and there are indeed some basic color divisions common throughout humanity, there must surely be a biological explanation.
Let’s start with the eye. People have three different types of cones at the back of their retinas: photoreceptors that are each most sensitive to particular frequencies of light. The S (short wavelength) cell is most sensitive to dark blue; M (medium) to green; and L (long) to yellow. (There are some exceptions to this rule: see box “The Mutants.”)
The fact that we humans have three types of cones is likely why we say there are three primary colors of light (blue, green, and red) from which most other colors can be made. These are only “primary” to us. “If a bird—who has four receptors—was running the world, it’s likely we’d have four primary colors,” said Skelton.
If it seems odd that the cones’ optimizations aren’t more evenly spaced across the rainbow, that’s because color isn’t their only job, pointed out Conway: our millions of photoreceptors also use light to map out everything we’re seeing, from text, faces, and trees to depth perception and more.
When it comes to color, “the cones are not very useful by themselves,” said Skelton. The real magic happens when the brain interprets the result. An initial step is to process the contrasting activation of different cones (a process called “cone opponency”). The details of how this happens are still disputed, noted Eskew. However it works, it’s not particularly intuitive (note for example that the three primary colors don’t match up with the three cone sensitivities). “Biology is intensely messy,” said Conway. The rods—photoreceptors best suited to seeing in dim light—might also have an underappreciated role in color vision, he said: “there’s a lot of exciting work going on with that right now.”
The brain then layers on all kinds of additional interpretations. It attempts, for example, to account for the effects of lighting. This is why a wall in sunlight is still perceived to be the same color as a wall in shade (even though a painter would use very different colors to represent them) and why there were so many disputes about whether that infamous 2015 viral meme photo of “the dress” was black and blue or gold and white (it depends on whether you thought it was in shadow or not).
Does all this basic biology result in some fundamental color categorization in our brains? It’s hard to peer into the brain to see. And doing studies on adults is problematic, since their raw brains’ wiring has likely been colored by language and experience over time. This idea is called the Sapir-Worph Hypothesis, and it has a lot of support and interest, if not a lot of hard data. One study found, for example, that native Greek speakers, who have different common words for light and dark blue, started to lose their ability to distinguish between these hues after spending a long time in England.
So, researchers turn to their best window onto the raw brain, unbiased by language or culture: babies.
Skelton thought that she might find up to four color categories in her babies’ heads. This is because of that “cone opponency” theory, which says that our brains first chart color on two primary axes: how red versus green something is, and how yellow versus blue (Skelton says it’s more accurate, based on how our brains chart different wavelengths, to say cherry/turquoise and chartreuse/violet; Conway calls it pink/cyan and lime/lavender). This provides a basic set of four distinctions.
But Skelton was surprised to find a fifth: a distinction between red and yellow. “This extra one is really interesting,” she said. Maybe the extra emphasis on “warm” color distinctions, she thinks, is because close-up objects are usually warm in color, while “cool” colors (blues and purples) more often appear in far-away vistas.
All of Skelton’s subjects grew up in the same part of England: Sussex, on the south coast (she didn’t collect information about their ethnic backgrounds). But other studies done elsewhere—including among the Himba—have also shown that babies categorize color in one way or another. Most of these studies only look at a few colors, rather than the whole spectrum, because the studies are so time consuming. For example, one study in Japan significantly looked at blood flow in the brain, instead of attentiveness, to show that babies distinguish between green and blue.
Skelton is convinced that these studies together imply that the five-fold basic color categorization is built into everyone, all around the world, and our languages evolve on top of that to represent whatever is most important in our culture. “It would be very surprising if the categories that we’d found here were unique to England,” she said.
But other interpretations are possible: Conway thinks these baby experiments can be subject to some of the same problems of the World Color Survey, where the observer’s biases get baked into the results. His own, as-yet-unpublished work, he says, hints that there may be far fewer basic pots of basic color, if any, in the brain. “I spent the better part of 15 years looking for [basic colors] in the brain and not finding them. All the evidence falls through your fingers like sand.”
The debate is sure to continue. A quick search reveals a plethora of papers with claims and counterclaims about color categorization data and its interpretations, as well as the misconceptions purportedly rife in the field. This is not a black-and-white issue—researchers still debate whether the methodology of these studies is being colored by their own inherent biases.
Whatever pots of colors may or may not be baked into their baby brains, as Skelton’s study subjects grow up, they will learn a native tongue from their parents. That will layer on top of their basic perceptions to shape how they think about and identify color. They’ll end up seeing the rainbow as their parents do. Or their perception will change and evolve with language and culture, giving them a new lens through which to see the world.
Extra: The Mutants
Although there may be some biological basis to the perception of color categories, not everyone is operating with the same biological bits and pieces.
Some people have defective or missing S, M, or L cones: each variant comes with a different set of impacts on color vision. The most common—a defective M cone—causes red/green color blindness, at a rate of about 5% in European people with XY chromosomes (those assigned male at birth). It’s also possible for one genetic mutation to wipe out the proper functioning of all three cone types in a single stroke, resulting in people who see only in shades of gray. The island of Pingelap is famous for an extremely high proportion of this condition.
Most of the genetic mutations that cause these variants are on the X chromosome, which means they tend to manifest in people with only one X chromosome—usually people with XY chromosomes. About 8% of people with XY chromosomes and 0.4% of people with XX chromosomes (those assigned female at birth) are color-blind in one way or another.
On the flip side of that coin, a person with XX chromosomes may have the luck of carrying genes for both a normal set of L cones and an altered set. These “tetrachromates” effectively have four types of cones, and thus seem to see colors slightly differently. They are sometimes called “super seers.”
A final variation is that some people can see past the end of the rainbow into the ultraviolet, just like bees, butterflies, and other insects. It turns out that the human cones and brain can detect ultraviolet light, but our lens usually filters it out. Some people who have had cataract surgery, in which the lens is either removed or replaced with an artificial substitute, start to “see” ultraviolet.
This happened to researcher Bill Stark, who suffered an eye injury when he was 10 and had cataract surgery at age 12. His new superpower later proved “convenient,” he said, for his academic interests in fruit fly vision. “What does UV light look like? Actually, it looks [like] a desaturated (whitish) blue,” he once wrote on his webpage. The same thing also happened to famous French painter Claude Monet, which is likely why his palette became bluer in later life.
This article first appeared in Atmos Volume 07: Prism with the headline “Over the Rainbow.”