Colour 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. It affects about 1 in 12 men and 1 in 200 women, making it one of the most common genetic conditions in the world. While the inherited form is by far the most frequent, colour vision can also be lost later in life through disease, medication, injury, or simply aging.
How Colour Vision Works
Your retina contains three types of cone cells, each built to detect a different range of light. One type responds best to red wavelengths, another to green, and a third to blue. Each cone contains a single light-sensitive protein called a photopigment that breaks down when it absorbs its target colour of light. That chemical reaction generates an electrical signal, and your brain combines the signals from all three cone types to produce the full spectrum of colour you perceive.
Colour blindness occurs when one or more of these cone types is missing, reduced in number, or contains a photopigment that doesn’t respond to the correct wavelength. The result is a gap or distortion in the colour information reaching your brain.
The Genetics Behind Red-Green Deficiency
The most common form of colour blindness, red-green deficiency, is caused by changes in two genes called OPN1LW and OPN1MW. These genes carry the instructions for building the photopigments in your red-detecting and green-detecting cones. When one of these genes is altered or missing, the corresponding cones either don’t develop or produce a photopigment that responds to the wrong wavelength of light.
Both genes sit on the X chromosome, which is why colour blindness is so much more common in men. Males have only one X chromosome, so a single faulty copy of one of these genes is enough to cause the condition. Females have two X chromosomes, meaning a working copy on the second X can compensate. For a woman to be red-green colour blind, both of her X chromosomes would need to carry the changed gene. This also means fathers cannot pass red-green colour blindness to their sons, since fathers contribute a Y chromosome, not an X, to male children. A colour blind father will, however, pass his affected X chromosome to every daughter, making her a carrier.
Types of Colour Vision Deficiency
Red-Green Deficiency
Red-green colour blindness actually covers four distinct conditions. Deuteranomaly is the most common type overall. It shifts certain shades of green toward red, though the effect is mild enough that many people don’t realize they have it. Protanomaly works in the opposite direction, making some reds appear greener and less bright. Both are considered mild and rarely interfere with daily life.
The more severe versions are deuteranopia and protanopia. In these conditions, one cone type is completely absent rather than just altered. People with either form cannot distinguish red from green at all.
Blue-Yellow Deficiency
Blue-yellow colour blindness is much rarer and follows a different inheritance pattern, since the gene for the blue cone photopigment is not on the X chromosome. Tritanomaly makes it difficult to tell blue from green and yellow from red. Tritanopia, the more severe form, removes the ability to distinguish blue from green, purple from red, and yellow from pink, and makes colours generally appear less bright.
Total Colour Blindness
Complete colour blindness, known as achromatopsia, affects roughly 1 in 30,000 people worldwide. It results from mutations in genes that control a signalling channel found exclusively in cone cells. When this channel doesn’t form properly, cones cannot convert light into electrical signals at all. People with achromatopsia see entirely in shades of grey and typically also experience extreme light sensitivity and reduced sharpness of vision. Unlike red-green deficiency, achromatopsia requires inheriting a faulty gene copy from both parents.
Diseases That Damage Colour Vision
Not all colour blindness is present from birth. A range of medical conditions can damage the cones, the optic nerve, or the brain regions involved in processing colour. Diabetes, glaucoma, and macular degeneration can all degrade colour perception by damaging the retina directly. Multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease affect the neural pathways that carry or interpret colour signals. Sickle cell anaemia, chronic alcoholism, and leukaemia have also been linked to acquired colour vision loss.
Acquired colour blindness often differs from the inherited kind in a few notable ways. It may affect only one eye, it can worsen over time as the underlying condition progresses, and it frequently shows up as a blue-yellow deficiency rather than a red-green one. Treating the underlying condition can sometimes partially restore colour perception, though this depends heavily on how much damage has already occurred.
Medications and Eye Injuries
Certain medications can alter colour vision as a side effect. Hydroxychloroquine, commonly prescribed for rheumatoid arthritis, is one well-documented example. The effect may be subtle at first, making it easy to miss without formal testing.
Physical damage to the eye can also cause colour blindness. Trauma from injury, surgery, radiation therapy, or laser treatment can disrupt the delicate structures in the retina responsible for detecting colour. Depending on the extent and location of the damage, the loss may be partial or complete, and it may affect one eye or both.
How Aging Affects Colour Perception
Even without disease, your colour vision gradually shifts as you age. The lens of your eye slowly thickens and accumulates yellow pigment over the decades, a process that eventually leads to cataracts in many people. This yellowing acts like a filter, disproportionately blocking short-wavelength (blue) light from reaching your retina. The result is a subtle but measurable loss of blue-yellow colour discrimination, which is why older adults sometimes struggle to distinguish navy from black or pale yellow from white.
Nuclear sclerotic cataract, the most common type, forms in the centre of the lens as part of normal aging. Its characteristic yellow-brown tint intensifies over time, progressively skewing colour perception. Cataract surgery, which replaces the clouded lens with a clear artificial one, often produces a dramatic improvement. Many people report that colours appear noticeably more vivid and blue-toned immediately after the procedure.
How Colour Blindness Is Detected
The most familiar screening tool is the Ishihara test, a set of dotted circle plates with numbers hidden in patterns of coloured dots. If you can’t see the number, you likely have a red-green deficiency. The test is quick and effective for red-green screening but cannot detect blue-yellow problems. A newer alternative, the HRR pseudoisochromatic test, matches the Ishihara’s accuracy for red-green deficiencies and adds plates designed to catch tritan (blue-yellow) defects as well.
Most inherited colour blindness is stable throughout life, meaning your results on these tests won’t change over the years. If you notice your colour perception shifting as an adult, that points toward an acquired cause worth investigating rather than a genetic one.