Where to Find the Colors Your Screen Can't Show You

Source: Hacker News

There are colors that I want to show you, but I can’t. They exist in the real world. You probably saw some of them today, but I can’t show them to you on a screen. A digital photograph can’t capture them, and your screen can’t display them. No game you’ve ever played has contained them. Unless you have specialized equipment, they are entirely absent from the digital world.

Most of them are cyans. On screens we live a life starved of cyans. It is shocking when you see one in person. They seem unfamiliar and intense in an otherworldly way. I want you to experience that, but again, I can’t show them to you. Instead, I have to show you how to find them in the real world.

“You sound like a crazy person, what are you talking about?”

(If colorspaces and the CIE chromaticity diagram are already familiar to you, you can skip to the next section.)

Light is made up of wavelengths, and its collection of wavelengths is called its spectrum. Your eyes have three different kinds of cone cells for seeing color, each of which respond differently to different wavelengths. Importantly, the cells in your eyes do not register what wavelength they are seeing. They can only respond, or not, with a certain intensity. Everything your brain figures out about the color of the world comes from contrasting the intensity of the responses of those cells.

Essentially all your cone cells can do is yell at your brain. Each of the cells wakes up and yells at your brain at a different volume, and that’s it. All your brain has available to work with to see color is how loud each of those cells are yelling, and has to reconstruct the whole rainbow from that alone.

A direct consequence of this is that any two spectra that make your cones all yell with the same pattern are indistinguishable to your brain. Even if the spectra contain entirely different wavelengths of light, to you they will look the same color. You don’t actually see light, not directly. You see how loud your cone cells yell.

Suppose color screens didn’t exist, and you were trying to design one for the very first time. The fact that we only have three different cones would seem very convenient. If you can figure out how to manipulate each of those three different cones independently, then your screen can make any human who looks at it see any color that a human can see. It doesn’t matter if it doesn’t show the real light spectra of real objects. All that matters is that the screen manipulates human cone cells, and can make them yell at human brains at different volumes. If you can do that, you’ve solved the whole problem. You might notice the suspicious coincidence between three cone cells and three primary colors. This is not a coincidence.

In 1931, CIE, (International Commission on Illumination) set out to characterize the whole space of human color vision. They produced this graph.

The outer rim of this graph shows every individual wavelength of light that humans can see. In the space enclosed by that rim are all the colors that can be produced with mixtures of those wavelengths. The points in this graph combine linearly, so if a color is in between two wavelengths, you can make that color by mixing those two wavelengths.

On this map they marked three wavelengths of light to be primary colors, and any color inside the triangle of those primary colors can be made by mixing them. The goal of these primary colors is to yank around your cone cells, and they picked these three because each of them yanks around one cone more than it yanks around the other two cones. This gives you pretty good control over a person’s eyes. You can almost make them see any color, but not quite.

Right away you see the problem. There’s a whole giant lobe of green/cyan/blue that can’t be made by mixing the primaries they chose. The green and blue primaries make one of your cones yell more than they’re supposed to. You can see this clearly on a chart of how to mix the primaries to make each wavelength. To make cyans that are cyan enough to be the most cyan thing we can see, you’d need to have negative red. Negative red doesn’t exist.

But wait, it gets worse. To make isolated pure wavelengths of light, CIE used prisms to scatter the light, followed by narrow slits to select a tiny band of a pure wavelength, a device called a monochromator. This is necessarily a big heavy bit of equipment that wastes most of its light, not something you would want to carry around in your pocket for a screen. When it came time to invent color TV, they didn’t use monochromators, they used phosphors. Phosphors don’t glow at pure wavelengths, so there was no physical way to push the primary colors on color TV to the edge of the chromaticity graph. Due to the limits of the phosphors they could make, we ended up with this.

That is, frankly, just not a lot of color. We have a much wider variety of light making technology available to us today. We have LEDs. We have lasers. We could do way better now. But CRT monitors displayed color with the same tech as color TVs, standards are standards, and most applications that use color are stuck inside that little window. This is called the sRGB gamut. Standard PC monitors, basically the whole internet, and mass market photography all live inside of sRGB. Critically for this article, matplotlib, the library I’m using to make graphs, only supports sRGB, so none of the colors outside of it will be represented in these graphs. Apple being Apple decided that wasn’t good enough so improved things a little bit.

This slightly wider triangle is standard now on essentially all smartphone screens regardless of manufacturer, all Macs, and most smartphone photos. Whether the content you’re viewing on the screens actually exercises the full color range is a different question, and is dependent on whether everything in the chain from the source to your eye preserved the colorspace.

It is not just our screens that are depriving us of cyans, it is also our lights. By unfortunate coincidence, the exact colors that screens can’t reproduce are also poorly reproduced by LED lighting. White LEDs are most commonly made with a blue LED and a yellow phosphor, and cyans fall right in the gap between the two. High CRI bulbs improve this by adding several different phosphors, but cyans are still the light they emit least.

It’s not enough to get off your screen, you’ll also have to go outside. Let me show you where.

Color Atlas

Natural Filters

When you look at a plant under normal light, its leaves are almost always within the sRGB triangle. Plants are green, but they aren’t that green. Their leaves absorb a lot of blue and red light, but not so much that it pushes us to the edge of the colorspace. The magic happens in a deciduous forest, when the light isn’t just reflected, it is transmitted. The transmittance curves of foliage are much more selective than their reflectance curves, so the color you see passing through a leaf is much more saturated than the color that bounces off of it. You’ve probably noticed this in person. A leaf lit by sunlight looks from the top to be relatively ordinary, but from underneath, it glows.

A single pass through a leaf knocks out all of the blues, and half of the reds, but the light then continues on, passing through other leaves, and bouncing off other leaves. These effects stack exponentially. The more times the light interacts with a leaf, the more it is purified to its spectral peak, generally around 550 nm. The colors you’ll see will be all the greens and yellows contained in the lobe traced out by the paths of repeated reflections and repeated transmissions. A green leaf lit by light that passes through another leaf one time is already outside of the gamut, greener than green.

When you’re standing in a maple forest at noon in the middle of summer, the intensity of the green is indescribable. Being in a fully lit and fully leafed deciduous forest is like being underwater if the water were green, which brings us to our next subject, water.

Water aggressively absorbs reds, slowly absorbs greens, and barely absorbs blues at all. This pattern pushes nearly any spectrum with blue and green in it out of the sRGB gamut almost immediately. When you look at sand in the shallow water near the coast, it traces a curve through colorspace as the depth of the water changes. The light from the sun is filtered once as it passes through the water on the way down, bounces off the sand, and filtered again as it comes back up to your eye. White or yellow sand will first shift to unrepresentable cyans, then to unrepresentable blues, and then finally converges close to the sRGB blue primary again once the water is very deep and dark.

But what happens if we combine water with a forest? Water in the wild isn’t just pure water, there are a lot of microscopic living things in it, and most of those little guys photosynthesize. They’re green just like leaves. Real water is like a mixture between pure water and a forest, and the density of phytoplankton in the water determines the path the spectra take as the water gets deeper.

When you are looking from above, the scattering of the light by the water itself and the particles in it begins to dominate the color of the sand. The depth of saturation the color can reach is limited, because mostly what you are seeing in deep water is light reflected back at you through just the top layers of water.

Just like in a forest, the real magic happens once you go inside of it, once you dive. If you are deep in the water itself, you are past the scattering, so the water and the plankton can repeatedly filter the light to their combined spectral peak before it arrives at your depth. You can fill nearly the whole gamut this way, but the BBC can’t show it to you on Blue Planet. It is more vivid than video can capture. Underwater photographers often use filters to block out blues, so that the whole scene doesn’t just clip against the limits of their sensor. These intensities of blues and greens are mostly unknown to the surface world, and beyond what we even have language to describe.

Note the commonalities of these processes. To get to the edge of the colorspace, they had to repeatedly filter light. Most natural materials are not so selective in their reflectance that their color includes none of the light on the opposite side of the color space, and that opposed light pulls the color in towards the center. It’s only by applying this process several times that the color is purified. There are however some processes in nature that are capable of this kind of filtering in one step, most commonly in birds.

Birds, Butterflies, and Structural Color

If I were writing this article for birds, It would be shorter to write about the inverse, the small set of bird colors that screens can show. Screens were designed for our mammal eyes, not for birds, and mammals, all mammals, can barely see color. We’re descended from tiny nocturnal scurrying things that lived during the Cretaceous. Our senses adapted accordingly. We have great noses, and good low light vision, but we lost most color vision. At night it’s just not worth differentiating the wavelength of a photon when there are barely any photons to go around. Better to indiscriminately absorb any scant few that make it into your eyes. Only primates have re-evolved the ability to tell reds from greens. Tigers are orange because deer, their primary prey, can’t tell the difference between tiger orange and grass green. Of the two colors, orange is easier for melanin to make, and to a deer, both orange and green are the color of grass.

Birds, however, are descended from big stomping dinosaurs that ruled the days, and have eyes perfectly adapted to the spectra of sunlight. The peak sensitivities of their cones are evenly spaced in the spectrum. They even have an independent cone for seeing ultraviolet light, which makes their fully saturated color space 3 dimensional. You cannot make a chromaticity diagram for a bird in a flat image. You’d have to make a chromaticity volume. A screen made for humans can’t even approximate the vision of birds. To them it would look like black and white with one added color.

The famous T-Rex Jurassic Park scene is totally implausible. It might be possible to salvage it by claiming that T-Rex had poor vision in low light, that is a competitive advantage of mammals, except that we also know that T-Rex’s eyeballs were some of the biggest in the animal kingdom, way bigger than an owl’s.

The quality of bird color vision has given birds much more reason than mammals to evolve vibrant colors for display. If mammals evolved vibrant colors, most other mammals couldn’t see them.

To make intense yellows, oranges, and reds, birds use the same chemicals, carotenoids, that make vegetables like tomatoes or carrots (eponymous) the same color. No animals can synthesize them themselves, so birds transfer them straight from their diets to their feathers, with sometimes just a little metabolism to shift their color on the way. To make blues and greens however, birds use an entirely different strategy, and this is the other reason for the intensity of bird coloration. Feathers also have a much wider variety of tools to make color.

We tend to think of light as a diffuse uniform field, or an abstraction, if we think about it at all, but real physical light has a length, and the length isn’t as small as you might think. The wavelengths of light you can see range from about ½ to ¾ of a micrometer, which is about 1/10th of the thickness of a strand of spider silk, or about 1/20th the thickness of plastic wrap. Light is small, definitely microscopic, but still similar to the size of real things.

The length of light determines where light can “fit.” Anything in nature that has patterns at around that scale can interact with light physically, not just chemically. You’ve seen this in the rainbows on a soap bubble, or in an oil slick. The liquid spreads out very thin, thin enough that it physically interacts with the length of light. Small variations in the thickness shift which colors it interacts with, which is how you get rainbows out of it. This is how birds make some of their most intense colors, especially those in the blue/green part of the spectrum.

Feathers are basically fractal hairs, as if a strand of hair grew hairs, and that hair also grew hairs, and then that hair also grew hairs. They have hairs on top of hairs, four levels deep. The first hair is the rachis, the shaft of the feather. The second hairs, the barbs, stick out laterally from the shaft, and are the smallest parts you can clearly see with the naked eye. Barbules extend sideways from the barbs, and zip together, using the fourth even smaller hairs, barbicels, as hooks that cling to each other when they are preened.

The barbs are too thick to do complicated things with light on their own, but the barbules are almost the right thickness. Birds that have flat omnidirectional color, like bluejays, make color inside their barbs by filling them with bubbles that are half the width of a wavelength. Birds that have iridescent colors, like hummingbirds or peacocks, make them in their barbules by stacking thin layers of dark brown absorbing melanin spaced half a wavelength apart. Light that is the right size can dodge the browns, but light that is bigger or smaller hits them and is absorbed.

Iridescent colors tend to be the most saturated structural colors. For a structure to be selective of the wavelengths that it reflects, light that hits it must always encounter gaps of the same width. It is difficult geometrically to do that in exactly the same way at every angle. From some directions the waves will fit nicely and reinforce each other. From others they will be askew, not fit, and be absorbed. Hence, iridescence.

Peacocks are a prototypical example. Using just the shape of the melanin layers in their barbules, peacocks can make half a dozen different colors. The blue on their chests and necks, and the cyan ringing the eyespots on their trains, are both outside the gamut, but all of these are made with the same dark brown pigments, spaced in layers to absorb all the photons whose length doesn’t let them weave between them. If you ground a peacock feather to powder, even if you were careful to only use regions of the same color, the result would be dark brown.

The eyespots on a peacock’s train are super cyan, so when the peacock spreads its train feathers it is going super saiyan super cyan.

There are around 500 species of birds with colors outside the sRGB gamut, and around 100 outside Display-P3. (The dataset I used was not exhaustive. There are probably more.) Some birds, like the male golden-tailed sapphire, a hummingbird from the western Amazon, have practically the whole spectrum in one bird.

Browsing the outer edges of this graph was a delight, and while I can’t show you what they truly look like through a photo or a screen, I want to at least show you the highlights and a suggestive hint of their appearance, which is all a photo can be.

Andean Cock-of-the-Rock, what a bird, what a name. He’d give you finger guns if only he had hands.

Emperor Fairywren. I like to think it is the chosen wren of the emperor of the fairies.

Paradise Tanager. When God was designing birds, this was the debugging bird He used to check whether all the parts of the bird were working correctly.

The quality of bird color vision didn’t just affect the coloration of other birds, it also affected the colors of their prey. Butterflies, in order to show off to birds that they are unpalatable or toxic, evolved iridescence dozens of separate times. The group of aptly named birdwing butterflies, with the largest wings of any butterfly, have the rare distinction of a species with an orange too orange for a Display-P3 screen. Ornithoptera Croesus has a color as rich as Croesus.

The scales on an iridescent butterfly’s wings are so complicated and varied that it is difficult to generalize between them, and even difficult to describe them as having a “color” rather than a range of colors in different circumstances. A single papilio palinurus butterfly can sweep across the colorspace from green to blue with different angles of view, or yellow to blue with different polarizations of light.

The morpho genus is perhaps the most famous, huge neotropical butterflies with a variety of intense blues and cyans between them and within them. I have a mounted specimen of morpho rhetenor. I can take a photograph of it, but in person it looks nothing like the photograph. I lack the words to describe how it differs from the photo except that it is somehow both more blue and more green.

Luminescence and Fluorescence

Deep in the ocean where there is no remaining light, animals have to make their own. The light they make could be any color, but water in the deeps still has the same absorbing properties that it does at the surface. If that glow is going to travel more than a short distance, it has to be blue or green. Creatures that glow cyan are abundant in the deep ocean, but sometimes the color comes to the surface. When the conditions are right, microscopic bioluminescent dinoflagellates bloom in surface waters in enormous numbers. If the night is dark enough to see it, they fill crashing ocean waves with the glow of the deep.

In some warm hypersaline lagoons, like on the island of Vieques in Puerto Rico, the conditions are always right, and anything dipped in the water at night, such as a kayak paddle, leaves a trail of cyan light behind it.

If you can’t catch a dinoflagellate bloom on the shore, or descend into the depths of the ocean, there are other species above water that glow with a similar color, but you still have to descend into the depths. In New Zealand caves, wherever the rocky ceilings stretch over water, they are speckled with cyan stars. The black pools of water below mirror the constellations above. These lights, despite looking like ocean bioluminescence, are made by glow worms, with an independent chemistry and evolutionary history. The worms make light to attract prey into their dangling mucus strands, which stretch up to two feet down from the ceiling, but are invisible in the dark. Better to keep the lights off.

There is another source of this color on land best seen in the dark. If you walk around any arid area at night with a black light flashlight, you may see things glowing cyan in the grass, things you may not have ever otherwise known were there, scorpions. Nearly every species of scorpion intensely fluoresces under UV light, with roughly the same teal as a dinoflagellate or glow worm glow.

No one knows for certain why. The primary theory is that it helps the scorpion see itself. Scorpions have photoreceptors in their tails, separate from their eyes. Scorpions also rely on hiding for their survival, lots of animals think of scorpions as a big tasty meal. It is hypothesized that a scorpion uses this fluorescence to tell whether any bit of its body is left exposed from its hiding place. Its tail “looks” down at its body, and if it sees its own fluorescence, it knows it is exposed to light, and in danger.

Man Made Color

“But Ryan,” you say, “I’m stuck at my desk right now. I can’t go to the beach, I can’t go to the woods, I can’t go out looking for tropical birds and butterflies, and I can’t go black light hunting for scorpions, as much as I would like to. Can’t you show me anything closer?”

You’re in luck. Today, on your way home, look at the “green” light on a traffic signal. It’s not green.

This may be the most acute Sapir-Whorf example I know of, that calling a “green” traffic light “green” was enough to make me ignore what my own eyes were telling me for my entire life. Green traffic lights are a beautiful indescribable turquoise, the most intense turquoise you’ve ever seen.

You’ll feel crazy once you see it, and want to run around telling everyone. Green traffic lights not only aren’t green, but they’re also exquisitely beautiful. My commute home the afternoon I learned about this was transcendent. I felt like my life suddenly had an entirely new sensation. How could I never have noticed? Green traffic lights are anti-memetic because you only stare at a traffic light when it’s red.

This is a good time to spare a thought for our red-green colorblind brethren. It is unlikely that any of them have read this far about a subject so alien to what they can experience, but it is to them that we owe the beautiful color of green traffic lights. The spectral requirements that make the green signals distinguishable from red in their eyes make them beautiful in ours.

The NIST standard for traffic lights has some tiny region of overlap with the display gamuts, but modern traffic lights are made with LEDs, and all LEDs (unless they have an added phosphor) make nearly pure spectral colors. This is probably the cheapest and most practical way to reproduce the whole colorspace. LEDs in spectral colors from one end to the other are readily available commercially.

While the band gap of LEDs admits only a very narrow range of wavelengths, there are some sources that are even more pure. Lasers are basically light duplicating machines. By energizing certain materials, a laser creates the conditions where one photon passing near an atom can cause an exact duplicate of the photon to be emitted. That duplicate goes on to create new duplicates by passing close to other atoms, in a chain reaction. Even if light enters the medium with a mixture of wavelengths, one wavelength will win out through this repeated duplication, and by the time the photons reach the other side, they are all exactly the same. So if you want to be absolutely certain you are seeing the purest most intense colors that it is possible to see, use lasers.

In all of my hunting, there was one region of the colorspace I was never able to fill with a naturally occurring color, my blue-green white whale. From what I’ve been able to find, no natural process emits 520 nm light at sufficient purity to make it close to the very top of the colorspace. Bioluminescent fungus peaks at around that wavelength, but the mixture of other wavelengths it emits causes its color to register far below.

This confused me for a long time until I realized it had a geometric explanation. At most of the positions at the edge of the color space, the spectral curve boundary is close to straight. An average of two points on a line always produces another point on the line, so in those regions a wider band of wavelengths doesn’t pull the color away from the edge, as long as it isn’t too wide. It is only when that band of frequencies passes the top of the curve at 520 nm and begins creeping down the other side that it pulls the chromaticity towards the center of the diagram. Extending far past 400 nm on one side or 700 nm on the other doesn’t desaturate the color, only crossing 520 nm in the center. This makes a color equivalent to the 520 nm point difficult for natural objects to produce. If the spectrum of an object is centered on 520 nm, any symmetric deviation from the peak immediately pulls the color away from the 520 nm point, and down into the center.

From this we can conclude what science fiction movies have understood intuitively all along. The most artificial color in the world, the clearest visual indication that you are interacting with advanced technology, is a green laser beam.

Qualia

At the end of all of this you might be wondering, “if I saw one of these, would I really notice? Is the difference actually apparent? Is this a genuinely new sensation, or maybe just a brighter version of what I am already familiar with?”

I can only speak for myself, but I noticed a very consistent pattern in my own sensations as I was searching for and studying color. I didn’t actually notice them, until I knew, and once I knew I couldn’t believe that I hadn’t noticed before. When you know what to look for, you attend to the sensations more closely, and they rise higher in your awareness than they otherwise would have. This is perhaps akin to what meditators report about their experience of their own self. When you ruminate on something you experience more of it.

The way we see the world isn’t just intermediated by screens. It is also intermediated by our own thoughts, what we notice and don’t, and what we think is important. In the same way that the designers of color standards had to make decisions about what sensations to reproduce and what to leave out, we are ourselves constantly triaging which of the demands on our attention are most important. The intensity of a color may not make the cut.

I can’t show you these colors, but by telling you about them I can help you notice them. When you notice, you may be astonished to find that they were there all along, and that your screens are duller than you thought they were. When you drive home today and see a green traffic light, notice it. Try to see it as bright and as beautiful as it really is.

But don’t bother taking a picture. It won’t work. Everyone else will have to see it for themselves.

Methodology and Acknowledgements

All colors of objects were rendered under the D65 standard illuminant using measured reflectance data. For data I could find in a repository, I used it directly. For data only present in a figure in a paper, I had Gemini 3.1 Pro extract it from the figure at 10 nm intervals, then plotted the extracted data to make sure it matched the original source without any gross errors. To find examples I started with hypotheses and then found spectral data to support them. There are likely many examples I didn’t find. In particular, I did not explore flowers, and did not explore synthetic pigments. (If anyone has a good data set to start from, I may add it later, or as part 2.)

The physical simulations of leaves and water I aimed to make naturalistic enough to be sure I was not misleading anyone about the intensity of colors that could be seen, without worrying too much about the exact physical circumstances where they could be seen. You might have to go deeper, or shallower, or in clearer or more fertile water than these graphs would indicate to achieve the colors depicted, but I tried to make sure I included all the important terms that might desaturate the color.

I would like to especially thank the colour python package for making this investigation possible, and the Bird Color Database for its wonderful collection which convinced me this project was doable and worth doing. And finally, I’d like to thank my family for putting up with me talking about color in every spare moment while on vacation.

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