Color blindness is caused by changes in the light-sensitive cells at the back of your eye, most often because of genes you inherited at birth. About 1 in 12 males and 1 in 200 females of Northern European ancestry have some form of color vision deficiency, with the vast majority being the red-green type. While genetics account for most cases, diseases, medications, aging, and even brain injuries can also disrupt color vision later in life.
How Your Eyes See Color
Your retina contains two kinds of light-detecting cells: rods, which handle dim light, and cones, which handle color. There are three types of cones, each tuned to a different range of wavelengths. Short-wavelength (S) cones respond most to blue light, medium-wavelength (M) cones to green, and long-wavelength (L) cones to red. Your brain determines what color you’re looking at by comparing signals across all three cone types. A ripe tomato, for instance, strongly activates red cones and barely activates blue ones, and your brain reads that pattern as “red.”
Color blindness happens when one or more cone types is missing, not working, or tuned to a slightly wrong range of wavelengths. Because a single cone can’t distinguish between a dim light at its peak sensitivity and a bright light at the edge of its range, your brain needs input from all three cone types to resolve the ambiguity. When one type is compromised, entire categories of color become indistinguishable.
The Genetic Cause Behind Most Cases
The genes responsible for building the red-sensitive and green-sensitive cone pigments sit right next to each other on the X chromosome. They share a high degree of similarity in their DNA sequences, which makes them prone to errors when genetic material is copied during reproduction. These errors can delete a gene entirely, fuse two genes together, or shift the wavelength sensitivity of a pigment just enough to cause problems.
Because these genes are on the X chromosome, the inheritance pattern hits males much harder. Males have only one X chromosome (paired with a Y), so a single defective copy is enough to cause color blindness. Females have two X chromosomes, meaning a working copy on the second X can compensate. For a woman to be red-green color blind, she’d need the defective gene on both X chromosomes, which is far less likely. This is why red-green color blindness affects roughly 8% of men but only about 0.5% of women.
The gene for the blue-sensitive pigment sits on chromosome 7, completely separate from the red and green genes. Defects in blue vision (called tritan defects) are not sex-linked and affect males and females equally, but they are much rarer.
Types of Color Blindness
Color vision deficiency isn’t one condition. It ranges from mild shifts in perception to a complete inability to see color, depending on which cones are affected and how severely.
- Deuteranomaly: The most common type overall. Green-sensitive cones are present but shifted in their response, making certain greens look more red. Most people with this type manage daily life without major difficulty and sometimes don’t realize they have it.
- Protanomaly: Red-sensitive cones are shifted, making reds appear more green and less bright.
- Deuteranopia and protanopia: Green or red cones are missing entirely. People with either type cannot distinguish red from green at all. These are the more severe red-green deficiencies.
- Tritanomaly: Blue-sensitive cones are altered, making it hard to tell blue from green and yellow from red.
- Tritanopia: Blue cones are missing, collapsing the differences between blue and green, purple and red, and yellow and pink. Colors also look less bright overall.
At the far end of the spectrum is blue cone monochromacy, an X-linked condition where both the red and green cones lose function, leaving only blue cones and rods working. People with this condition have significantly reduced visual sharpness, very limited color perception, and sometimes develop progressive changes in the central retina over time. Complete achromatopsia, where none of the cone types work, is even rarer and results in a world seen essentially in shades of gray.
Diseases That Damage Color Vision
Not all color blindness is present from birth. Several eye diseases can impair color perception by damaging the retina or the optic nerve. Glaucoma, age-related macular degeneration, diabetic retinopathy, cataracts, and retinitis pigmentosa all fall into this category. The color loss from these conditions tends to develop gradually and may affect one eye more than the other, which is a key difference from inherited forms that are usually equal in both eyes.
Neurological conditions can also interfere. Multiple sclerosis, Parkinson’s disease, Alzheimer’s disease, and strokes can disrupt the pathways between the eye and the brain or damage the brain regions that process color. In rare cases, bilateral strokes in a specific area of the lower back of the brain (the inferior occipitotemporal region) cause cerebral achromatopsia, a complete loss of color perception even though the eyes themselves are healthy.
Medications and Chemical Exposure
Certain medications can alter color vision as a side effect, sometimes temporarily and sometimes permanently. Hydroxychloroquine and chloroquine, widely used for autoimmune conditions like lupus, can damage the retina with long-term use and impair color discrimination. Ethambutol, a tuberculosis drug, can affect the optic nerve and shift color perception. Digoxin, a heart medication derived from the foxglove plant, is historically associated with a yellow-tinted visual disturbance. Erectile dysfunction medications like sildenafil can cause a temporary blue tint to vision by affecting photoreceptor signaling, though this usually resolves within hours.
If you take any of these medications regularly, periodic color vision screening can catch changes early, before they become irreversible.
How Aging Changes Color Perception
Even without disease, your color perception naturally shifts as you get older. The culprit is the lens inside your eye. Over decades, the normally clear lens undergoes a process called brunescence: it gradually yellows and becomes denser, blocking more light from reaching the retina. Short-wavelength (blue) light is filtered out most heavily. Studies comparing younger and older observers found that older adults’ lenses absorbed roughly ten times more light at the blue end of the spectrum (around 400 nanometers) than younger lenses did.
The practical effect is that blues become harder to distinguish from greens. When researchers simulated aged-lens filtering for younger viewers, those younger participants used the word “blue” less often and started calling blue-ish colors “green” instead. This age-related shift is subtle and progressive, so most people adapt without noticing. It’s not the same as inherited color blindness, but it can compound the effects if you already have a mild deficiency.
How Color Blindness Is Diagnosed
The most familiar screening tool is the color plate test, where you look at circles filled with colored dots and try to identify a number or shape hidden inside. People with normal color vision see the figure clearly, while those with a deficiency see a different number, no number at all, or a faint shape. This test is quick and effective for detecting red-green deficiencies.
For jobs that demand precise color discrimination, like graphic design or electrical work, eye doctors use a hue test. You’re given a set of colored blocks and asked to arrange them in rainbow order from red to purple. Errors in the sequence reveal both the type and severity of any deficiency. Together, these two tests can pinpoint whether you have a protan, deutan, or tritan defect and how much it affects your daily color judgment.