Red and green color blindness is formally called “red-green color vision deficiency,” and it falls into four clinical subtypes: protanomaly, protanopia, deuteranomaly, and deuteranopia. The “protan” types involve problems with red-light perception, while the “deutan” types involve problems with green-light perception. About 8% of men and 0.4% of women of European descent have some form of it, making it by far the most common type of color vision deficiency.
The Four Subtypes Explained
Your eyes contain three types of cone cells, each tuned to a different slice of the light spectrum: red (long-wavelength), green (middle-wavelength), and blue (short-wavelength). Red-green color blindness happens when either the red-sensitive or green-sensitive cones malfunction or are missing entirely. Which cones are affected, and how severely, determines which of the four subtypes you have.
Protanomaly: The red-sensitive cones are present but shifted slightly out of alignment, so they respond weakly to red light. You still use all three cone types, but reds look duller and harder to distinguish from greens and browns. This is the milder “protan” form.
Protanopia: The red-sensitive cones are completely nonfunctional. With only two working cone types, you cannot perceive red light at all. Reds may appear as dark gray or muddy brown, and telling red from green becomes extremely difficult.
Deuteranomaly: The green-sensitive cones are present but contain a pigment that’s shifted toward the red end of the spectrum, making it too similar to the red cone pigment. You still have three working cone types, so color vision is only partially affected. This is the single most common form of color blindness.
Deuteranopia: The green-sensitive cones are completely absent or nonfunctional. Like protanopia, this leaves you with only two cone types and a significant loss of color discrimination in the red-green range.
The two milder forms (protanomaly and deuteranomaly) are classified as “anomalous trichromacy,” meaning you still mix three types of light to see color, just not in the normal proportions. The two severe forms (protanopia and deuteranopia) are classified as “dichromacy,” meaning you rely on only two cone types. Dichromats need just two primary colors to match any color in the spectrum, where a person with normal vision needs three.
Severity Varies Widely
Even within a single subtype, the degree of impairment can differ dramatically from person to person. Among people with deuteranomaly, for example, some have color discrimination that’s nearly normal, while others struggle almost as much as someone with full deuteranopia. The variation depends on how similar the two remaining cone pigments are to each other. When the pigments overlap heavily, distinguishing colors in the red-green range becomes much harder. Clinicians sometimes subdivide anomalous trichromacy into “simple” and “extreme” categories to capture this range.
Why It Runs in Families
Red-green color blindness is inherited through the X chromosome. The genes that produce the red and green cone pigments sit right next to each other on the X chromosome, and they share more than 98% of their DNA sequence. Because they’re so similar, they’re prone to a copying error during cell division where sections of one gene get swapped with the other. This shuffling can delete a gene entirely (causing dichromacy) or create a hybrid gene that produces a pigment tuned to the wrong wavelength (causing anomalous trichromacy).
This X-linked pattern explains why men are affected so much more often than women. Men have only one X chromosome, so a single defective copy is enough to cause color blindness. Women have two X chromosomes, so a normal copy on one can compensate for a defective copy on the other. A woman would need to inherit the defective gene from both parents to be affected, which is relatively rare. She can, however, be a carrier and pass the trait to her sons.
Prevalence Across Populations
Large population surveys put the prevalence at roughly 8% of men and 0.4% of women among people of European descent. In men of Chinese and Japanese ethnicity, the rate is lower, between 4% and 6.5%. The ratio of affected men to affected women also differs between ethnic groups. More recent surveys suggest prevalence is rising among men of African ethnicity and in regions with significant migration and population mixing, likely because gene flow introduces more variation into the opsin gene array.
What Colors Actually Get Confused
The name “red-green color blindness” is somewhat misleading. It doesn’t mean you see the world in black and white, or that you only confuse red with green. The affected range is broad. People with protan deficiencies have trouble with any color that contains red: reds can look brownish or dark, oranges may blend with yellows, and purples can be hard to tell from blues because the red component is invisible or faint. Greens and browns become easy to mix up as well.
Deutan deficiencies cause a similar but not identical pattern. Greens, yellows, oranges, and certain light reds tend to blur together. The specific pairs that cause confusion depend on the subtype and severity, but common trouble spots include traffic light colors, the difference between ripe and unripe fruit, color-coded charts and maps, and the red or green indicators on electronics. People with mild anomalous trichromacy may not even realize they see colors differently until they take a screening test, while those with dichromacy often discover the issue early in childhood when they consistently name colors incorrectly.
How It’s Detected
The most familiar screening tool is the Ishihara test, a set of circular plates filled with colored dots that form numbers or patterns. People with normal color vision see one number; people with red-green deficiency see a different number or none at all. The Ishihara test is good at detecting whether a deficiency exists, but it doesn’t precisely classify the subtype or severity. For that, eye care providers use more detailed instruments like an anomaloscope, which asks you to match two colored lights by adjusting their mix. The specific match you choose reveals whether you have a protan or deutan deficiency and how severe it is.
There is currently no cure for inherited red-green color blindness, since the underlying cone cells are either missing or permanently altered. Specially tinted glasses and contact lenses can enhance contrast between certain colors for some people, though they don’t restore normal color vision. Most people with the condition adapt well, relying on brightness, context, and position cues (like the placement of lights on a traffic signal) rather than color alone.