In reality, very few people are truly color “blind,” and unable to see any color. Most who have difficulty differentiating colors have what is more accurately described as color vision deficiency, or CVD.
Because CVD is one of the more commonly inherited conditions, and because it can be acquired through illness, disease or accidents, it affects many people around the globe. Sometimes the deficiency is subtle and may not be noticeable. For others, difficulty differentiating between red and green, for instance, can lead to troubles on the road or in the classroom.
Color vision deficiency may seem minor, but it’s not something to take lightly, says Wadih Zein, a staff clinician at the National Eye Institute, part of the National Institutes of Health. “It’s important for people to know about this disease–especially teachers and educators who deal with young people,” he says. Teachers can make adjustments—as simple as changing colors used for bar graphs—that can make a big difference for someone with CVD, who might otherwise struggle with learning, says Zein.
Humans perceive color through the image-processing retina, a thin layer of two types of light-sensitive cells at the back of the eye: rods, which help detect images in low light; and cones, which work in daylight and are responsible for color differentiation. Color is a physical property of light—each color represents a different wavelength. That’s easily seen when light is refracted through a prism. But human cone cells only see red, green and blue light. The brain interprets signals from the cones to form the perception of various colors.
For instance, if yellow light is shining into the eye, the green and red cones are activated because yellow is close to both of those colors. The brain receives the signals from those cones and you see yellow. You will also see yellow if the green and red cones see green and red light; when those colors overlap, they are perceived as yellow.
But that’s normal vision. It’s different for people with CVD.
CVD is the result of defects on the genes that give instructions to the cone cells. Those genes are found on the X chromosome, which makes CVD a more commonly inherited condition in men, but women also can inherit a deficiency if the defect is present in both X chromosomes. It can show up at birth or later in life.
An unknown number of people inherit a CVD. Zein says the best-known statistics come from those who have been most-studied: Caucasians of Northern European ancestry. The National Eye Institute estimates that as many as eight percent of men and 0.5 percent of women with that genetic background have the common form of red-green color blindness, which is broken into several types.
About one percent of men have an abnormality just in the red cone cell, which means red, orange and yellow appear greener, and colors are not as bright. In another one percent, the red cones don’t work at all, which means red looks black, and some shades of orange, yellow and green appear as yellow. If the green cells don’t work—a condition called deuteranopia, which affects one percent of men, also—green looks beige, and reds are brownish-yellow.
The most common form of a CVD overall is deuteranomaly, in which the green cone is abnormal. Yellow and green look red, and it is hard to distinguish to violet from blue. Some five percent of men have the condition.
Blue-yellow color blindness, in which blue cones (tritan) are completely missing or have limited function, is less common. When blue cones are missing, blue will look green and yellow will be violet or light gray. If the blue cones are barely functioning, blue might appear to be green. Both conditions affect men and women equally.
The most severe forms of CVD include monochromancy, which means no cone cells work, and people only see black, white and gray, and blue cone monochromancy, in which only the blue cone cells work. About one in 100,000 boys have it at birth. It can cause reduced visual acuity, near-sightedness and nystagmus (uncontrollable eye movements).
A color vision deficit isn’t always inherited. Sometimes it develops after an illness or injury that damages the retina or the optic nerve, says Zein. Even cataracts can occasionally cause color vision disturbances, such as a mild loss of contrast or hues becoming more yellow, he says.
Not everyone sees color in exactly the same way. Still, “color vision studies indicate there is a range of color that can be perceived by individuals,” Zein says. Some will have more trouble distinguishing hues—say between the blue and the black socks in the drawer.
Many people with CVD go through life unaware of their color issues. But for some, it can interfere with some things the rest of us take for granted—like being able to color coordinate outfits or differentiate the red, green and yellow on a traffic light.
“It also limits your choices as far as careers are concerned,” Zein says, noting that jobs that require pinpoint differentiation among colors would be a challenge for someone with a CVD.
The good news is that CVD does not get worse over time. The bad news: it can’t be cured, and so far, scientists have not been able to come up with a treatment.