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

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.