Color blindness
Based on Wikipedia: Color blindness
The World Through Different Eyes
John Dalton, the brilliant chemist who gave us atomic theory, spent his entire life seeing the world differently from almost everyone around him. Red hunting jackets looked green to him. Scarlet academic robes appeared indistinguishable from the grass beneath his feet. When he finally published a paper about his condition in 1794, he became one of the first scientists to describe color blindness from personal experience. Today, in many languages, the condition is still called "daltonism" in his honor.
But here's what might surprise you: Dalton wasn't rare. Not even close.
Roughly one in twelve men—about eight percent—experience some form of red-green color blindness. That's approximately the same percentage as left-handed people. If you're designing a website, creating a traffic system, or choosing which wire to cut to defuse a bomb in an action movie, you're potentially confusing millions of people.
How Color Vision Actually Works
To understand color blindness, you first need to understand how color vision works at all. Your retina—the light-sensitive tissue at the back of your eye—contains two types of photoreceptor cells: rods and cones. Rods handle low-light vision and don't process color. Cones, however, are your color detectors.
Most humans have three types of cone cells, each sensitive to different wavelengths of light. Scientists designate these as L-cones (long wavelength, roughly red), M-cones (medium wavelength, roughly green), and S-cones (short wavelength, roughly blue). When light hits your retina, these three cone types respond in different combinations, and your brain interprets those signals as the full spectrum of colors you perceive.
This three-cone system is called trichromacy. It's why computer screens and televisions can create virtually any color by mixing just red, green, and blue pixels. Your visual system evolved to be hackable.
Color blindness happens when one or more of these cone types doesn't work properly—or doesn't exist at all.
The Genetic Lottery
Here's where it gets interesting from a biological standpoint. The genes that code for your L-cones and M-cones sit on the X chromosome. This single fact explains almost everything about who gets color blindness and who doesn't.
Women have two X chromosomes. Men have only one, paired with a Y chromosome. If a woman inherits a defective color vision gene on one X chromosome, her other X chromosome usually has a working copy that can compensate. She becomes a carrier, typically with normal color vision, but capable of passing the trait to her children.
Men don't have this backup system. One defective gene on their single X chromosome, and there's no redundancy. The trait expresses fully.
This is why color blindness affects about eight percent of men but only half a percent of women. For a woman to be color blind, she needs to inherit defective genes from both parents—a much rarer occurrence that requires both a color blind father and a carrier or color blind mother.
The Red-Green Confusion
When most people hear "color blindness," they imagine a world of pure black and white. That's almost never the case. Complete absence of color perception—called monochromacy or total color blindness—is extraordinarily rare, affecting perhaps one in thirty thousand people.
The vast majority of color blind individuals have red-green color blindness, which comes in two main flavors with Greek-derived names that sound like medical conditions from a Victorian novel.
Protanopia means the L-cones (red-sensitive) are missing or non-functional. The name comes from Greek: "protos" meaning first, plus "anopia" meaning blindness. The L-cone is considered the "first" cone type in the scientific naming convention.
Deuteranopia affects the M-cones (green-sensitive). "Deuteros" means second in Greek.
Both conditions produce remarkably similar experiences of the world. People with either type struggle to distinguish red from green, orange from yellow-green, and brown from certain greens. Purple looks blue. Pink might look gray. The traffic light problem is obvious.
But there's a crucial difference between these two types. People with protanopia don't just confuse red—they see it as significantly darker than normal. A bright red fire truck might appear almost black. Red traffic lights can seem dim or even extinguished. This dimming effect doesn't occur in deuteranopia, where green is affected but red maintains its apparent brightness.
The Milder Forms
Complete absence of a cone type represents one end of the spectrum. The other end is much more common and much more subtle.
Protanomaly and deuteranomaly are the "anomalous trichromacy" versions of these conditions. People with these variations still have all three cone types, but one type doesn't respond to quite the right wavelengths. Their L-cone or M-cone is shifted slightly in sensitivity, overlapping more than it should with the neighboring cone type.
The result? They're still trichromats—they still have three-cone vision—but the color matches they make differ from people with normal vision. Show them a specific shade of yellow, and they'll need more red or more green in a red-green mixture to match it than a normal observer would.
Many people with mild anomalous trichromacy don't even know they have it. They've never seen the world any other way, and their color vision is close enough to normal that daily life presents few obvious problems. They might struggle occasionally with certain color combinations or find that their spouse disagrees about whether that shirt is gray or green, but these quirks often go undiagnosed for decades.
The Forgotten Type: Blue-Yellow
Red-green color blindness dominates the conversation, but there's a third type that plays by different rules entirely.
Tritanopia and tritanomaly affect the S-cones, the blue-sensitive receptors. The "tritos" in the name means third. Unlike red-green color blindness, this form isn't sex-linked—it sits on chromosome seven, not the X chromosome. Men and women are affected equally.
It's also exceptionally rare, occurring in less than one in ten thousand people. And when it does occur, it's more often acquired than inherited—caused by eye disease, brain injury, or certain medications rather than genetics.
People with tritanopia see blues and violets as greenish and dramatically dimmed. Some short-wavelength colors appear almost black. Yellow becomes indistinguishable from white. The world shifts toward a red-green axis, with the blue-yellow dimension collapsed.
The Complete Absence of Color
True monochromacy—seeing the world only in shades of gray—does exist, but it's vanishingly uncommon and typically comes with other serious visual problems.
Rod monochromacy, also called complete achromatopsia, occurs when the retina develops no functional cones at all. Only the rod cells work. These individuals see in grayscale, yes, but they also struggle intensely in bright light. Rods evolved for dim conditions; in daylight, they're overwhelmed. People with rod monochromacy often experience severe light sensitivity, involuntary eye movements, and significantly reduced visual acuity—typically around 20/200 or worse.
Blue cone monochromacy is slightly less severe. One cone type remains functional, providing some ability to see in normal light conditions, but still no color discrimination. These individuals typically have visual acuity in the 20/50 to 20/400 range and frequently develop nearsightedness.
There's also an acquired form called cerebral achromatopsia, where the eyes work fine but the brain can't process color information. This usually results from stroke or brain injury affecting specific regions of the visual cortex. Patients can sometimes distinguish colors in tests—their retinas still respond differently to different wavelengths—but they consciously perceive only grayscale.
Living in a Color-Coded World
Consider how often modern life relies on color as the primary or only way to convey information. Traffic lights. Ripe versus unripe fruit. The subtle pink flush of a sunburn developing on a child's shoulders. The red warning light on a dashboard. The green "online" indicator next to a colleague's name. Whether meat is still raw in the center.
Color blind individuals develop coping strategies so automatic they barely notice using them. Traffic light position gets memorized: top is always stop, bottom is always go. Bananas get judged by firmness rather than the green-to-yellow transition. Cooked meat gets tested with a thermometer rather than visual inspection.
But some situations resist workarounds.
Many professional fields legally exclude color blind individuals. Aircraft pilots need to distinguish colored aviation lights and read color-coded instruments. Train drivers face similar requirements. Police officers, firefighters, and military personnel often must pass color vision tests. Electricians need to identify wire colors correctly—the stakes for getting red and green mixed up can be lethal.
The Confusion Colors
Specific color pairs predictably confuse people with each type of color blindness. These "confusion colors" aren't random; they fall along mathematical lines in color space that converge at specific points for each condition.
For red-green color blind individuals, the classic confusions include cyan with gray, rose-pink with gray, blue with purple, yellow with bright green, and the entire red-orange-brown-green cluster. Hand someone with deuteranopia a standard set of crayons, and they might genuinely struggle to sort the greens from the browns.
Tritanopes face different confusions entirely: yellow with gray, blue with green, dark violet with black, and red with rose-pink. Their color world collapses along a different axis.
Seeing What Others See
Scientists use an instrument called an anomaloscope to precisely diagnose and measure color vision deficiency. The test is beautifully simple: the subject adjusts a mixture of red and green light until it matches a reference yellow light. Normal trichromats converge on a specific mixture. Protanomalous observers need more red. Deuteranomalous observers need more green. Dichromats can match the yellow across a wide range of red-green mixtures because their visual system can't distinguish between them.
The more common screening test is the Ishihara test—those dotted circle images where people with normal vision see numbers hidden in colored dots, while color blind individuals see nothing, a different number, or sometimes numbers invisible to normal observers. It's named for Shinobu Ishihara, a Japanese ophthalmologist who developed it in 1917 to help screen military recruits.
The Search for Solutions
There is no cure for inherited color blindness. The cone cells simply develop differently, and you cannot retrofit a retina.
But research into gene therapy shows promise for certain severe conditions. Scientists have successfully used viral vectors to deliver functional color vision genes into the retinas of monkeys with color blindness, effectively giving adult animals cone types they'd never had before. Human trials remain limited, but the principle has been demonstrated.
In the meantime, companies like EnChroma sell glasses with specialized filters designed to help red-green color blind individuals distinguish colors more easily. The lenses work by selectively blocking certain wavelengths of light, effectively increasing the separation between the signals from L-cones and M-cones.
The marketing often shows emotional videos of people "seeing color for the first time." The reality is more nuanced. These glasses don't create new cone cells or restore normal vision. They can help with certain color discrimination tasks, making some previously confusing color pairs more distinguishable. But they don't allow wearers to see colors they couldn't perceive before—they enhance the contrast between colors the wearer already sees, just not well.
Smartphone apps offer another form of assistance, using the camera to identify and name colors on demand. Point the phone at something, and the app announces "forest green" or "burgundy." It's not the same as seeing the color, but it provides the information.
The Upside of Difference
There's ongoing debate about whether color blindness affects artistic ability. The conventional assumption would be that it's purely a disadvantage, but some researchers suggest it might confer certain perceptual advantages.
Color blind individuals often have superior ability to detect camouflaged objects. When color cues are unreliable or deliberately misleading, the brain focuses more on texture, shape, and luminance patterns. During World War II, some military units specifically recruited color blind soldiers to spot camouflaged enemy positions that would fool normal-sighted observers.
Several famous artists are believed to have been color blind, though posthumous diagnosis is always speculative. The painter Charles Meryon produced masterful engravings but reportedly abandoned color work due to his inability to see it properly. Some art historians have suggested that certain Impressionists' unusual color choices might reflect atypical color vision rather than purely stylistic decisions.
Design in a Color Blind World
If you're designing anything that uses color to convey information—websites, charts, traffic systems, warning lights, product packaging—the statistics matter. With eight percent of men affected by red-green color blindness, any significant user base includes substantial numbers of people for whom your red-green distinctions are invisible.
Good accessible design never relies on color alone. The text "click the green button" fails if someone can't identify which button is green. Red error messages disappear if the red and the surrounding text are indistinguishable. Charts become unreadable when the legend requires distinguishing red lines from green ones.
The fixes are often simple. Use both color and another indicator: shape, position, pattern, or text label. Make the red button square and the green button round. Put the warning message in a bordered box, not just red text. Add texture or different line styles to chart elements.
British Rail figured this out decades ago. Their signals use dramatically different colors than American systems—a blood red that's almost purple, an amber that's clearly yellow, and a green that's distinctly bluish. The colors are chosen specifically to maximize distinguishability for color blind operators.
Traffic lights themselves contain built-in redundancy through position. Top is always stop, bottom is always go. Even if you can't tell red from green, you can tell top from bottom. The system works because it doesn't depend entirely on color.
A Different Way of Seeing
There's something philosophically interesting about color blindness. Every person with normal color vision assumes the colors they see are "real"—that the red they perceive corresponds to something objective in the world. But color is constructed by the brain from wavelength information. Different brains construct it differently.
A rare few people have provided remarkable insights into this construction. Some individuals have one normal eye and one dichromatic eye—usually due to disease affecting only one eye. These "unilateral dichromats" can directly compare what dichromatic vision looks like against normal vision. They consistently report that, with their color blind eye, wavelengths shorter than a certain neutral point appear blue, and wavelengths longer than that point appear yellow. The reds and greens collapse into variations on these two colors.
Mantis shrimp have sixteen types of color receptors. Butterflies have five. Dogs have two. Humans, with our three cone types, occupy one arbitrary point on a vast spectrum of possible color vision systems. Color blindness isn't a failure to see reality correctly. It's just another way of constructing color from the same underlying physical information.
John Dalton requested that his eyes be examined after his death to determine the cause of his color blindness. In 1995—over 150 years after he died—DNA analysis of his preserved eye tissue revealed he had deuteranopia. His M-cones had never developed properly. The world he experienced, the one that led him to investigate and describe this condition that still bears his name, was a world without green.
But it was still a world full of color. Just different colors. Just a different way of seeing.