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Colour Blindness Test: How It Works, Types and Symptoms to Know
Published on: Sep 08, 2026
TABLE OF CONTENTS
Most people who are colour blind do not know it. The condition rarely causes complete loss of colour, more often it produces a specific, quiet gap in colour discrimination that goes undetected for years. Colour blindness tests are designed to expose exactly that gap: they present stimuli that a person with normal colour vision resolves easily but that a person with a colour vision deficiency cannot. Understanding how these tests work is useful both for those who suspect a problem and for those being screened as part of an eye examination or occupational assessment.
What Is Colour Blindness?
Colour blindness, more precisely called colour vision deficiency (CVD), is the reduced ability to distinguish between certain colours. In most cases it is not an absence of colour perception, but specific hues appear similar or indistinguishable from one another.
The condition arises from a defect in the cone photoreceptors of the retina. The human retina contains three types of cone cells: L cones (sensitive to red), M cones (sensitive to green) and S cones (sensitive to blue). Colour blindness results when one or more cone types are absent, present in reduced numbers, or contain a photopigment with a shifted spectral sensitivity.
Colour vision deficiency is significantly more common in men because the L and M cone photopigment genes are located on the X chromosome. Women, with two X chromosomes, are usually carriers rather than affected.
Symptoms of Colour Blindness
Symptoms of colour vision deficiency are often subtle and may not be identified without specific testing.
Difficulty distinguishing red from green: Affected individuals may confuse red and green traffic lights, struggle to identify ripeness in fruit, or fail to spot red text or diagrams on a green background.
Difficulty distinguishing blue from yellow: People with tritan defects confuse blue and yellow and may mistake yellow for white or pale green.
Washed-out or dull colour perception: Many people with CVD describe colours as less saturated or vivid than those seen by individuals with normal colour vision. Reds may appear brownish, greens may appear yellowish or greyish.
Difficulty reading colour-coded information: Maps, pie charts, electrical wiring diagrams and chemistry indicators present practical difficulties for people with red-green CVD.
Problems in specific occupations: Colour vision is required for roles including airline pilot, train driver, naval officer, electrician and some surgical specialties.
Childhood academic difficulties: Colour-coded school textbooks for science, geography, and mathematics rely heavily on colour differentiation. Children with undiagnosed CVD may underperform in these subjects without the underlying reason being identified.
Types of Colour Blindness Tests
They are:
Ishihara colour test: The most widely used screening test for red-green colour blindness worldwide. The test consists of a series of pseudoisochromatic plates, each showing a number or pattern formed by dots of a particular colour against a background of dots in a confusable colour. A person with normal colour vision reads the embedded number but a person with red-green CVD cannot. The Ishihara test is highly sensitive for red-green CVD but does not test for blue-yellow defects.
Farnsworth-Munsell 100 Hue Test: Requires the patient to arrange a series of coloured caps in order of hue across the visible spectrum. It identifies the specific colour axis of the defect and its severity. The Farnsworth D-15 is a shorter version used for occupational screening.
Anomaloscope: The gold standard for diagnosing and classifying red-green colour deficiency. The patient matches a yellow reference light by adjusting the ratio of red to green in a mixture field. The anomaloscope distinguishes definitively between normal trichromacy, anomalous trichromacy, and dichromacy.
City University Colour Vision Test: Uses pseudoisochromatic plates similar to the Ishihara test but includes blue-yellow plates, allowing detection of acquired colour deficiencies as well as congenital ones.
How to Test for Colour Blindness at Home
Several validated online colour vision screening tools allow preliminary self-assessment at home, including digitised versions of the Ishihara plates. The most commonly used free online versions include the EnChroma colour blind test and Colourlite.
Home testing has important limitations. Screen calibration, ambient lighting, and display resolution all affect colour rendering significantly. An Ishihara plate on a poorly calibrated screen may give a false-normal result. Online tests are useful for identifying a likely colour vision deficiency but are not a substitute for formal testing under standardised illumination with printed plates or a calibrated anomaloscope.
Understanding the Colour Vision Chart
The colour vision chart most people encounter clinically is the Ishihara chart. Each plate in the set is constructed using the principle of pseudoisochromacy: the target (a number or path) and its background are designed to be isoluminant (equal in brightness) but differ only in colour. Because luminance cues are removed, only true colour discrimination can distinguish the target from the background.
The plates are read under standard illuminant D65 (daylight-equivalent illumination) at a distance of 75 centimetres. In the 38-plate set, a score of 17 or more is considered normal. Scores below this threshold indicate a red-green colour vision deficiency and the pattern of errors identifies whether the defect is protan (red axis) or deutan (green axis) and its approximate severity.
Transformation plates, showing one number to people with normal colour vision and a different number to people with CVD, are also included in the full Ishihara set. Plates accessible only to people with CVD are included as further discriminators.
Hard Colour Blind Tests: What They Reveal
The most demanding colour vision assessments, used in specialist occupational and clinical settings, go beyond screening for the presence of CVD to characterise its type and severity precisely.
The Farnsworth-Munsell 100 Hue Test is considered the most difficult standard colour vision test because it requires discrimination across the entire hue circle, not just the red-green axis. Errors on the FM100 are plotted on a polar diagram that reveals the colour confusion axis specific to the individual, and the test also detects acquired colour vision loss from retinal disease (including AMD, diabetic retinopathy and glaucoma).
The anomaloscope provides the most precise clinical data. The Rayleigh match (red-green axis) and Moreland match (blue-green axis) are the standard tests. The Rayleigh equation discriminates definitively between protanopes and deuteranopes with a precision no plate test achieves.
Can Colour Blindness Be Treated or Corrected?
Congenital colour blindness caused by an inherited cone photopigment defect cannot currently be cured. However, several approaches can partially compensate for or reduce the impact of CVD.
Colour-correcting spectacle lenses and contact lenses: Tinted lenses that selectively filter specific wavelengths can increase contrast between colours on the red-green confusion axis. EnChroma glasses use a notch filter to separate the overlapping L and M cone spectral sensitivities. These lenses do not restore normal colour vision but may allow better discrimination in some individuals with anomalous trichromacy. They are not effective for dichromacy.
Adaptive technology: Smartphones and computers offer accessibility features for people with CVD including colour filters, colour-identification apps and browser extensions that remap confusable colour pairs to distinguishable ones.
Occupational and educational accommodation: For many people with CVD, early identification and appropriate accommodation in school and the workplace is the most practical intervention.
Conclusion
Colour blindness is a common, largely hereditary condition that significantly affects daily function and occupational eligibility when undetected. The Ishihara chart is the most practical clinical screening tool in India, while the anomaloscope and FM100 provide definitive diagnosis. Home testing is a useful first step but must be followed by formal optometric assessment. Early identification through school screening remains the most effective intervention.
FAQs
What is a colour blindness test used for?
A colour blindness test detects and characterises colour vision deficiency. It is used for clinical diagnosis, occupational screening (aviation, railways, armed forces), school screening, and monitoring acquired colour vision loss from retinal disease.
What are the symptoms of colour blindness?
Difficulty distinguishing red from green or blue from yellow, washed-out colour perception, difficulty reading colour-coded materials and problems identifying ripe fruit or coloured text. Symptoms are often subtle and unnoticed without testing.
How does the red-green colour blind test work?
The Ishihara test uses pseudoisochromatic plates in which a number is formed by dots of one colour against a background of a confusable colour. A person with red-green CVD cannot read the embedded number.
How can I test for colour blindness at home?
Online digitised versions of the Ishihara plates provide preliminary screening. Screen calibration and lighting significantly affect accuracy. Home tests indicate likely CVD but not a substitute for formal testing with printed plates under standardised illumination.
What is the colour vision chart used in eye tests?
The Ishihara chart is the most widely used colour vision chart. Each plate uses a pseudoisochromatic design to isolate colour discrimination from luminance cues. Results are scored as the number of plates correctly identified.
What is the number-based colour blindness test called?
The Ishihara test is the number-based colour blindness test. Each plate shows a number embedded in a field of differently coloured dots. The standard set contains 38 plates.
Can colour blindness be corrected or treated?
Congenital colour blindness has no cure. Colour-filtering spectacle lenses can improve discrimination of some colour pairs in anomalous trichromats. Gene therapy trials are underway. Occupational accommodation is the most practical current intervention.
Is colour blindness more common in men or women?
Colour blindness is significantly more common in men because the most common forms are inherited through the X chromosome.
What causes colour blindness?
The most common cause is an inherited mutation in the genes encoding the L or M cone photopigments on the X chromosome. Colour vision deficiency can also be acquired through retinal disease (AMD, diabetic retinopathy, glaucoma).
How accurate are online colour blindness tests?
Online tests are useful for preliminary screening but are limited by screen calibration, ambient lighting and display resolution. They identify likely CVD but occupational assessments require formal testing with standardised printed plates or anomaloscopy.
Is Colour Blindness Hereditary?
Yes. Red-green CVD is caused by mutations in the OPN1LW and OPN1MW genes on the X chromosome, inherited in an X-linked recessive pattern. Men are affected when they carry one mutated X chromosome. Women are usually carriers. Tritanopia (blue-yellow CVD) is inherited in an autosomal dominant pattern. Achromatopsia is autosomal recessive.
References
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