How Color Blindness Works

Dorothy's ruby slippers wouldn't have been the same in black-and-white.
Photo by MGM Studios/Courtesy of Getty Images

Roses are red, violets are blue -- well, bluish. The sky is blue, too. Grass is green. These are things that most of us know for a fact and don't question. But what if you were colorblind? What would you see? Is life one long black-and-white movie?

In "The Wizard of Oz," Dorothy Gale steps out of her black-and-white Kansas farmhouse and into the color-saturated Land of Oz. She moves from a humdrum existence of chores and troubles to an intense fantasyland peopled with curious creatures, trading in a clapboard house for yellow brick roads, red ruby slippers and a brilliant green city of emeralds. What would her transformation have been without this rainbow of colors?


Color isn't just a component of vision. We associate color with beauty, like in a gorgeous sunset. Some colors have meaning in and of themselves -- purple is for royals, red signifies passion. Colors seep into our expressions -- If we're depressed, we say we're feeling blue. We're also "green with envy," we "see red" and we might go "white with fear." Colors even have practical meaning -- red means stop, green means go.

Certain colors are said to help you sleep, while others make you hungry. And never underestimate the effect of a bright red dress. Color is important.


Types of Color Blindness

Retina of eye showing rod cells (photoreceptors) x600. Tinted B&W scanning electron micrograph.
Ron Boardman/Riser/Getty Images

To understand colorblindness, you need to understand some things about color vision.

The part of the eye that senses light and deals with color vision is called the retina. There are structures in the retina shaped like rods and cones -- rods help to see in low light, cones help to see in color and also with seeing details. Rods and cones contain photosensitive chemicals. In rods, this chemical is rhodopsin. The chemicals in cones are called photopigments. There are three kinds of cones, and each cone has a different photopigment that's sensitive to a certain wavelength of light. Because most of us have all three kinds of cones, normal human vision is called trichromatic.


Colorblindness is a misleading term. It makes everything sound black and white. Color vision deficiency might describe the condition more clearly. There are different kinds of color vision problems and different degrees of severity. Red-green color vision defects are the most common.

People who have mild color vision defects have anomalous trichromacy, which means that they do have all three types of cones, but one of the cones is defective. Someone with deuteranomaly, the least severe kind of colorblindness and also the most common, has unusual red cones, while someone with protanomaly has unusual green. People with deuteranomolous vision may not even know they don't see colors normally. Tritanomaly, trouble distinguishing blue and yellow, is quite rare.

People who are missing one type of cone altogether have dichromatic vision, which is more serious than anomalous trichromacy. Within this category of dichromatic vision, we have three different types.

  • Deuteranopia: no green cones (also referred to as L cones, as in sensitivity to long wavelengths of light)
  • Protanopia: no red cones (M or medium wavelength cones)
  • Tritanopia: no blue cones (S or short wavelength cones)

Monochromacy is the next step on the color vision ladder. Monochromats see life in black, white and shades of gray. There are two types of monochromacy: rod monochromacy and cone monochromacy. People with rod monochromacy, also called achromatopsia, also have very poor vision and a high sensitivity to light. They also have nystagmus, which makes your eyeballs look kind of wobbly.


What the Color Blind See

How do you see through someone else's eyes? It's difficult to say how someone else sees color, because it's so subjective. How do I know that what I see as red is what you see as red? Is my red richer and bolder than yours? Or faded and dim compared to what you see? You may have argued with someone over whether a pair of pants is navy blue or black and never quite seen eye to eye.

When you refer to a rainbow of colors, you think ROYGBIV: red, orange, yellow, green, blue, indigo, violet. So what does someone who's colorblind see? Nothing that varied and exciting.


With this brightly colored image, we'll look at how deuteranopia and protanopia (both types of red-green colorblindness) as well as tritanopia affect how someone sees.

Image courtesy Henrik Sorenson/Stone/Getty Images

Deuteranopia: The reds are all gone. The woman's dress appears greenish-gray, and the purple has left her umbrella. The green from the umbrella looks rather grayish. Even her skin has lost its pinkish tone.

Protanopia: Looks a lot like deuteranopia, doesn't it? Except that the red dress has gone to a darker gray.

Tritanopia: The yellow in the image has turned to a light pink. There's no orange. The blue of the sky has changed and so have all the reds. What you're mostly seeing looks like shades of pink and blue. It's duller.

Anomalous trichromats have trouble telling green, yellow and red apart. They may see no difference at all between the purple object you're holding up and the blue one (since the purple involves red light.)

A monochromat sees black, white and gray, and what he sees is probably rather blurry.

On the pros side, people who have mild red-green color deficiency are said to be better at detecting camouflage. Same with dichromats -- they're more attuned to texture, instead of being fooled by the patterns of color [source: Gene Reviews].


Testing for Color Blindness

Color blindness test plates
Dorling Kindersley/Getty Images

The kind of colorblindness test you're probably most familiar with is Ishihara plates and pseudoisochromatic plates (PIP). Ishihara plates were developed by Dr. Shinobu Ishihara for the Japanese army. His original plates were hand-painted in watercolor and showed Japanese characters. Now, an Ishihara plate shows an assemblage of color dots with a number in the middle made with different colored dots. Ishihara plates can help diagnose red-green color vision defects. It isn't the perfect test, though -- sometimes the colors in one set don't quite match up with the plates in a different set, or maybe they look different in one kind of lighting than in another.

The Navy, Marines and Coast Guard use the Farnsworth Lantern test, aka FALANT, to diagnose colorblindness. The FAA uses it as a backup test if someone fails the PIP test. The FALANT involves colored signal lights that have to be identified from a distance. Two lights are shown at once, and the person being tested must identify the color (red, green, or white). The lights, however, are darkened with a filter to prevent people who are colorblind from being able to tell colors apart by their brightness. All dichromats and almost all anomalous trichromats fail [source: Gene Reviews].


The most accurate test for diagnosing different types of colorblindness is the anomaloscope. The person being tested has to match colors by controlling the brightness of a yellow light on part of their screen and a mixture of red and green light on the other side. He adjusts these elements until both sides of the screen appear to have the same color and brightness. People with normal vision match things very precisely, while people with color vision deficiencies match things imprecisely.

Causes of Color Blindness

Magnified retina of healthy eye and retina of diabetic eye (mottled)
UHB Trust/Stone/Getty Images

Where does color blindness come from? How does it happen? Like so many things, it's mostly about your genes -- the luck of the draw.

It's true that many more men than women are colorblind. About 8 percent of Caucasian men have red-green color deficiency, compared to .5 percent of women. Colorblindness is less common in African and Asian populations. Achromatopsia, complete colorblindness, affects one in 40,000 people. On the Pingelap islands, however, where marriage to relatives is common, achromatopsia occurs in 5 to10 percent of the population. Tritan defects affect fewer than 1 in 10,000 people around the world.


Red-green color blindness is sex-linked recessive -- it's carried on the X chromosome. Men have an X and a Y chromosome, so if the X chromosome carries the gene mutation for colorblindness, he's going to be colorblind. Women have two X's, so they'd have to get two copies to make it happen.

Tritanopia isn't sex-linked -- it's an autosomal dominant disorder, but it can also be acquired. Most acquired colorblindness falls under tritanopia. Glaucoma, for instance, can cause blue-yellow colorblindness. Men and women get tritanopia in equal numbers. Complete colorblindness is autosomal recessive.

Some drugs, like digitalis and chloroquine, can even cause colorblindness. So can some industrial chemicals, as well as injuries to the eye.

Here are some diseases and conditions that can cause color vision defects:

  • diabetes
  • glaucoma,
  • macular degeneration
  • Alzheimer's
  • Parkinson's
  • alcoholism
  • leukemia
  • sickle cell anemia
  • multiple sclerosis

[source: Mayo Clinic, HealthLink]

Aging also has an effect on color vision. Colors seem to fade as we get older.

Unfortunately, there's no cure for colorblindness. There are some corrective lenses that on the market that claim to help with colors, but they can mess with depth perception and other aspects of visions. With any luck, the future will hold surgical options or perhaps gene therapy, but for now, coping mechanisms are the best bet.


What the Color Blind Can Do

Color pleases us aesthetically and serves as a visual clue, but do we really need it? It's a question worth thinking about.

When an architect thinks about designing a building, he or she ensures that it's accessible to people with wheelchairs and tries to make allowances for people's handicaps. Does anyone take the colorblind into account?


Perhaps since it's not a condition that's visible (only visual), people have a hard time understanding it. And after all, you can't explain how you see. How would you explain green to someone who's never seen it? You could try to do it with verbal descriptions or with music, but it wouldn't be the same.

There are daily frustrations for people who are colorblind. They may have a limited color palette in their wardrobes, to avoid showing up on a first date in a bright purple shirt and mismatched pants. For a kid, crayons without labels are pretty much useless. Most frustrating, perhaps, is listening to other people talk about beautiful, colorful things and not being able to see them.

Colorblindness can also be an obstacle to certain careers. If you've seen "Little Miss Sunshine" (spoiler alert if you haven't), a boy's dreams of becoming a pilot are abruptly dashed when he realizes he's colorblind. It's true -- you can't be a pilot if you're colorblind. There are too many signals to catch that rely on color to keep other people safe. You might also have trouble being an electrician (matching color-coded wires), and a woman at a salon might not want you coloring her hair. If you search the Internet for colorblindness and careers, you'll come up with a whole list of can'ts.

But let's take graphic design. You might immediately write that off as a career for the colorblind. But not so fast. First of all, a good Web site design won't rely just on colors to highlight important elements. The designer will use other elements to set off the things that are most important -- different fonts, contrast. Second, colors on the Internet correspond to hexadecimal values. Blue isn't written in code as "blue" -- instead, it's #0000FF. So someone who's colorblind could potentially keep in mind the color wheel and remember which numbers match with which colors.


Lots More Information

Related HowStuffWorks Articles

More Great Links

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