Color blindness

Colour blindness, also known as colour vision deficiency (CVD), is a condition characterised by a reduced ability to perceive colour or by unusual differences in colour perception. The severity of the condition ranges widely, from mild difficulties that are barely noticeable to a complete inability to perceive colour. In most cases colour blindness arises from inherited variations affecting the cone cells of the retina, although it can also result from disease, injury or ageing.

Causes and Types of Colour Blindness

Congenital red–green colour blindness is the most common form and encompasses protan and deutan variations. It affects approximately one in twelve males and one in two hundred females owing to the X-linked inheritance pattern of the relevant opsin genes. Rarer forms include tritan (blue–yellow) deficiencies, blue cone monochromacy and achromatopsia, the latter involving near-total absence of colour perception.
Acquired colour blindness may result from damage to the retina or optic nerve, neurological disorders or exposure to certain chemicals and medications. Natural ageing also leads to gradual deterioration in colour discrimination.

Diagnosis and Management

Diagnosis typically involves specialised colour vision tests, most widely the Ishihara plates, which identify red–green deficiencies. Other tests assess functional colour discrimination, particularly in occupations where accurate colour recognition is critical.
There is no cure for most inherited forms, although research in gene therapy has shown promise for future treatment of severe deficiencies. Many affected individuals develop intuitive coping strategies, and mild forms often have limited impact on daily life. Colour-corrective glasses, such as those marketed by EnChroma, may improve performance in some specific colour tasks for certain red–green deficiencies, but they do not restore typical colour vision. Mobile apps that identify colours using a device camera can also assist users.
Certain professions—including aviation, rail transport, firefighting, policing and military service—may restrict entry for individuals with significant colour deficits because colour signals and safety indicators are integral to the work.

Effects on Colour Perception

People with colour blindness have reduced or absent discrimination along the red–green axis, the blue–yellow axis or both, depending on the type of deficiency. Individuals usually discover their condition through mistakes in identifying or naming colours, particularly when young. Each type of colour blindness has predictable confusion colours, which are pairs or groups that appear similar to those with the same deficiency.
Examples include:

• Red–green deficiencies: confusion between cyan and grey, blue and purple, yellow and green, and various greens with red, orange or brown.
• Tritan deficiencies: confusion between yellow and grey, blue and green, violet and dark tones, and red with rose hues.

These confusion patterns can be represented mathematically by confusion lines on chromaticity diagrams such as the CIE 1931 colour space. For individuals with dichromacy, colours lying on the same confusion line may appear metameric, provided they are adjusted to equal lightness.

Colour-Dependent Tasks

According to classification by Cole, colour tasks fall into four types, all of which may be affected by colour blindness:

• Comparative: tasks requiring direct comparison of hues, such as mixing paint.
• Connotative: tasks involving meaning associated with colour, such as “red means stop.”
• Denotative: tasks that demand naming or identifying colours.
• Aesthetic: tasks involving harmony or emotional impact of colour combinations.

Daily activities affected include:

• Food: difficulties judging ripeness (e.g., bananas), detecting bruising or rot, determining when meat is cooked, or distinguishing flavour-coded colours in confectionery.
• Skin: subtle changes in complexion, bruising, sunburn or rashes may be overlooked.
• Traffic lights: distinguishing red, amber and green can be challenging, especially when lights appear close to white or when sodium streetlights distort hues. Positional cues are therefore essential.

Special design measures, such as shape-coded signals in parts of Canada or distinctive colour tones in British railway signals, can make navigation safer for those with deficiencies. Aviation and maritime navigation rely on red–green lights, and functional testing such as the Farnsworth Lantern Test is frequently required for safety-critical roles.
Colour analysis in fashion and personal styling, which depends heavily on subtle aesthetic cues, can also be more difficult.

Art, Creativity and Colour Blindness

Colour blindness does not preclude artistic achievement. Several prominent artists, such as Clifton Pugh and Charles Méryon, adapted their styles or media and enjoyed successful careers. In modern animation, designer Jin Kim has also demonstrated that red–green colour blindness does not hinder creative accomplishment.
Debate continues over whether colour blindness limits artistic expression, but many artists employ texture, contrast and composition rather than fine colour discrimination.

Possible Advantages

Research suggests that colour blindness may offer certain perceptual benefits. Individuals with deuteranomaly may better distinguish shades of khaki, potentially helpful for detecting camouflaged objects. Dichromats, who rely more on texture and form, may be less deceived by camouflage designed for observers with normal trichromatic vision. Some studies indicate that people with certain deficiencies can discern colour differences that typical observers cannot, possibly explaining the persistence of red–green deficiencies across populations.
During the Second World War, colour blind observers were sometimes used to detect camouflaged enemy positions, taking advantage of these perceptual differences.