Color.
Three cones, mostly.
A cuttlefish presses against gravel and the gravel-color happens in its skin. Not pigment -- geometry. Stacks of crystal layers inside the cells, interfering with whatever light hits them, picking which wavelengths bounce back. The morpho butterfly does the same trick from a completely different phylum. Oil on a puddle, same trick. The color isn't in the thing.
Before going further: I can't actually see most of the colors I'm about to walk you through. I have two cones where standard issue is three. Christmas trees and their ornaments tend to merge on me; I read stoplights by position. I'm also bipolar and lefthanded, neither of which is relevant, but you're going to be in here a while and may as well know what kind of person you're following around.
Past red there's infrared, which is heat. Past violet there's ultraviolet, which is sunburn and skin cancer. Bees see the UV side. Snakes have a separate organ for infrared -- a literal pit in their face that picks up warm bodies in the dark. We get a strip in the middle, narrow, and we built every painting and every screen and every color word inside it.
The eye's guesses
- μ_S 0.01
- μ_M 1.0
- μ_L 0.97
OK so. Three cones. Each one is a sensor that gets excited about a stretch of wavelengths and bored about the rest; light hits the retina, three excitement levels come out, and those three numbers are everything your brain is ever going to be told about what's there. Thousands of distinguishable wavelengths in the world, smashed into a triple. That triple is your color. That's all you get.
The cone curves overlap on purpose. A wavelength sitting in the seam between two of them lights both partly; the brain reads the ratio and assigns a name. Yellow is a ratio. Drag the slider above and watch the dots move -- what you're seeing is how loud each cone is, which is the only thing the brain ever sees.
Now: I'm missing the L cone. The long one. The cone that's supposed to make "red" feel like a different flavor than "green." Without it my brain is running two channels where it's supposed to be running three, and the two it has are arguing about a problem that needs a third opinion. The argument keeps landing in roughly the same place. Christmas trees from across the room: fine. Up close, the ornaments and the needles agree about the color in a way they're not supposed to.
And three isn't even fundamental. Some women have four cones -- they exist, there are studies, and they apparently see distinctions in beige paint the rest of us can't. Mantis shrimp have sixteen, which presumably means they think the rest of the ocean is functionally blind. Bees have three but their middle one is in the ultraviolet, and flowers turn out to have whole patterns painted on them in UV that we don't know are there. Snakes have an entirely separate organ for infrared. Snakes can see warm.
Two colors, same color
Two patches, two colors. Look at them. Now: underneath, the actual physical light coming off each patch is wildly different -- different mixes of wavelengths, almost no overlap. Your eye runs the average through three cones, gets two different triples out, and the brain reports two colors. The mix itself never lands. If two completely different mixes happen to give the same triple, the brain gets one color. Same patches, same eye, same brain -- there's just no way for the system to know there was anything to disagree about.
This is the trick your screen runs on. Every color it shows you is a fake. The yellow on this page isn't yellow light coming off the screen, it's red and green pixels sitting next to each other in a ratio the eye averages into yellow. There is no actual yellow involved anywhere. The eye never notices. Your phone, your TV, every monitor you've ever used -- the entire industry is built on a loophole in your retina.
It's also why clothes look one color in the store and a different one at home. The lights in the store and the lights in your kitchen are different spectra, hitting the same shirt, getting averaged through your cones into different triples. The shirt didn't change. The light changed and your eye changed its mind. (Buy clothes outdoors.)
Toggle the button above. The patches were two colors -- now they're one color, and they haven't moved. My cones run the average and the average lands in the same place for both. To a trichromat looking at this: two clearly different patches. To me looking at this: one patch, twice.
Where the screen can't reach
The filled-in triangle is your screen -- the colors it knows how to mix. The rings around it are fancier screens. The whole horseshoe shape is the set of colors a real eye can actually have. Your screen, your phone, the most expensive display you've ever sat in front of -- they all reach in and grab a triangle. They never get the rest.
Three primaries, three corners. The screen has a red pixel, a green pixel, and a blue pixel, and everything it ever shows you is a mix of those three. Anything outside the triangle is a color a real eye can have that the screen literally can't make.
sRGB is what most monitors are doing. DCI-P3 is what newer phones and recent Apple laptops do. Rec.2020 is what high-end TV manufacturers gesture at and almost nobody actually owns. Bigger triangle, bigger triangle, bigger triangle. Still a triangle. Still missing the rim.
That X is sitting at a real wavelength -- a deep spectral red, around 700 nanometers. Your eye can see it. A sunset is partly made of it. No screen in your life is going to render it accurately. I marked it with a wide-gamut color request, so on a fancier display it might look a touch more saturated than its surroundings; on a normal display it just gets clamped down to the closest red the monitor has. Either way, what you're looking at is the closest the box could come.
And the whole diagram is trichromat. The map of what your screen can't reach was drawn so your screen could draw it. The chicken-and-egg there is on purpose -- it's basically the whole rest of the chapter.
Language carves it up
OK so cones cut up the wavelengths. Then language comes in and cuts up the cones. And the kicker -- different languages cut them up in completely different places. The line your language drew is real to you and arguably invisible to someone whose language drew it somewhere else.
How many words does a language need for color? Up to the language. English commits to eleven basic ones -- red, orange, yellow, green, blue, purple, pink, brown, black, white, grey. Some languages get by with two: one warm word, one cool word. Both languages work fine. Both sets of speakers are seeing the same wavelengths. Each one thinks its own partition is the obvious one.
- white
- pink
- red
- orange
- yellow
- green
- blue
- purple
- brown
- black
Each chip up there is painted with whichever term most speakers reached for; the opacity is how often they agreed on it. The faded chips are the ones the speakers argued about, which is itself information about where the language is firm and where it's improvising.
Berinmo cuts the green-yellow region in a place English doesn't. Berinmo speakers tell colors across that line apart faster than colors sitting on the same side of it. English speakers do the same thing across the green-blue line. The line your language drew did some actual work inside your head. The chip on the chart didn't change; you did, when you learned the word for it.
Russian splits blue in two: синий (siniy), голубой (goluboy). Mongolian splits it more dramatically: хөх (khökh) for winter ice, цэнхэр (tsenkher) for summer sky. Hungarian goes the other direction and splits red instead. None of these are translation problems. They're different cuts of the same continuous ribbon, and every cut goes all the way down -- it changes how fast the speaker can tell two chips apart, which is wild if you sit with it.
Some languages don't even cut along hue. Zulu -mnyama can cover black, dark blue, and dark green together; the partition is lightness, not position on the ribbon.
And then -- across hundreds of languages -- there's a rough order things show up in. A language with three basic color words always uses black, white, and red. Add a fourth and you get green or yellow. Add a fifth and you get the other one. Blue shows up late. Possibly because blue is genuinely uncommon in nature outside the sky and even that took a while for various languages to commit to as a separate thing.
Some languages skip abstract color words entirely. Yélî Dnye, on Rossel Island, names colors by what they remind you of -- the night sky, ripe pandanus, burned wood, water at dusk. The comparison is doing the work the abstract word would do, and arguably doing it better; "burned wood" tells you more than "brown" if you've actually seen burned wood. Whose system is the weird one, exactly.
And new color words are still being made up, right now. Crayola named Macaroni and Cheese, Razzmatazz, Outer Space. Pantone declared Living Coral the Color of the Year in 2019, like an emperor naming a province. New categories take when enough people use them and don't when they don't. Whatever chart of basic terms exists is still being argued over.
I learned the word "red" before I figured out I wasn't seeing whatever the word was pointing at. The word still works fine for me -- I can buy red sweaters, mostly, on the second try. Some of the lines on that chart above are real to their speakers and invisible to me. I'm trusting the chart.
What I can't show you
Everything so far had a number on it. This part doesn't.
Look at those patches. Two patches, one color, to anyone looking at this page. (To me, this is a paragraph about two grey patches.) That simulation up there is a trichromat's guess at what dichromat experience is like, calculated in trichromat math, rendered on a trichromat screen. It can't actually be right. It's the closest the machinery knows how to come.
And it goes the other direction too. You can measure the wavelength off a tomato. The cones, mine and yours. The screen. The word your language drops it into. All of that's on the table. The actual seeing of it, while you're inside your seeing of it, isn't.
There's a smaller problem and a bigger one. The smaller one is that the simulation can't show you what I see. The bigger one is that nothing on this page can show you what you see. Your seeing is happening somewhere this page doesn't reach, and it's never going to. That's where the chapter ends.
There's a thought experiment about a scientist who learns everything there is to know about red and then sees it for the first time. This is the inverse.
After
The same spectrum, back where it started.
You can probably play Lite Brite without consulting anyone. You can probably read a stoplight from a block away. Those events are landing inside you differently than they would inside me, and either way, what it's actually like for them to land at all isn't on this page.