Color blindness is determined through a series of visual tests that range from quick screening plates to precision instruments that pinpoint the exact type and severity of a deficiency. The most common first step is a pseudoisochromatic plate test, where you identify numbers or shapes hidden in dots of color. But screening is just the beginning. Depending on why you need an answer, whether for personal curiosity, a child’s development, or a career requirement, different levels of testing apply.
How Color Vision Works
Your retina contains three types of cone cells, each sensitive to a different range of light wavelengths: short (blue), medium (green), and long (red). No single cone can detect color on its own. Each cone simply counts how many photons it absorbs, regardless of wavelength. Your brain determines color by comparing the activity levels across all three cone types. When one type of cone is missing, altered, or produces a pigment with a shifted sensitivity range, that comparison breaks down, and certain colors become difficult or impossible to distinguish.
The genes for red and green cone pigments sit next to each other on the X chromosome, which is why red-green color blindness is far more common in men. About 8% of men of European descent and 4% to 6.5% of men of Chinese and Japanese descent have a red-green deficiency. In women, the rate drops to roughly 0.4%, because they carry two X chromosomes and a functional copy on one can compensate for a faulty copy on the other. Blue cone deficiency, controlled by a gene on chromosome 7, is much rarer and affects men and women at similar rates.
People with color vision deficiency fall into two broad categories. Dichromats are missing one cone pigment entirely, so they rely on only two cone types. Anomalous trichromats have all three cone types, but one produces a pigment whose sensitivity is shifted toward another, making fine color distinctions harder without eliminating them completely.
The Ishihara Plate Test
The Ishihara test is the most widely used first-line screening tool for red-green color deficiency. You look at a series of circular plates made up of colored dots. Within each plate, a number or path is formed by dots that differ in color but match in brightness, so someone with normal vision reads the number easily, while someone with a red-green deficiency sees a different number or none at all.
The standard passing threshold is 12 correct responses out of 14 red-green test plates (not counting a demonstration plate). Research shows this cutoff has 97% sensitivity and 100% specificity, meaning it catches nearly everyone with a deficiency and almost never flags someone who sees color normally. Accuracy depends on proper lighting, consistent timing, and clean test plates. Smudges, dim lighting, or hints from the person administering the test can all skew results.
The Ishihara test has one significant blind spot: it does not detect blue-yellow deficiencies. If that type of deficiency is suspected, a different screening tool is needed.
HRR Plates for Blue-Yellow Deficiency
The Hardy-Rand-Rittler (HRR) plate test works on the same principle as Ishihara, using colored symbols embedded in fields of dots, but it includes plates specifically designed to detect blue-yellow (tritan) deficiencies alongside red-green ones. An updated version of the HRR, released in 2002, improved sensitivity for mild tritan deficiencies. In direct comparisons, subjects with mild blue-yellow deficits who tested as normal on the original HRR made detectable errors on the newer edition. If your eye care provider suspects a tritan issue, HRR plates are a better screening choice than Ishihara.
The Farnsworth-Munsell 100 Hue Test
Screening plates tell you whether a deficiency exists, but they don’t measure how severe it is. The Farnsworth-Munsell 100 hue test fills that gap. You arrange 85 colored caps (despite the name, there are not exactly 100) into a smooth gradient between two fixed anchor caps. The test measures how accurately you place each cap in sequence.
Errors are tallied into a total error score: the more caps you misplace, and the farther off they are, the higher the score. The pattern of errors also reveals which axis of color discrimination is affected, red-green or blue-yellow, making it useful for both congenital and acquired deficiencies. Clinicians and researchers also use it to track changes in color discrimination caused by neurological disease, retinal conditions, or medication side effects over time.
The Anomaloscope: The Gold Standard
The Nagel anomaloscope is considered the definitive diagnostic instrument for red-green color vision deficiency. It works by asking you to match colors rather than identify hidden patterns. You look into an eyepiece and see a small circular field split in half. One half shows a yellow light. The other half shows a mix of red and green light that you can adjust. Your task is to blend the red and green until the two halves look identical.
A person with normal color vision accepts only a very narrow blend, roughly 55% red and 45% green, to match the yellow. The range of mixtures you accept is scored on a scale from 0 to 73, and the width of that range directly reflects the severity of your deficiency.
The test also distinguishes between different types of deficiency with precision. Someone with a mild green-deficient shift (deuteranomalous) needs less red in the mixture than normal, accepting a broader range on the lower end of the scale. Someone with a mild red-deficient shift (protanomalous) needs more red, accepting a range on the higher end. A person who is fully dichromatic, missing one cone pigment entirely, will accept any mixture across the full 0 to 73 range, simply adjusting the brightness of the yellow to match. Protanopes (missing red cones) also perceive long-wavelength light as dimmer, so they set the yellow reference light to a lower brightness when the mixture contains more red. Deuteranopes (missing green cones) do not show this brightness shift, which helps clinicians separate the two.
Anomaloscopes are expensive and typically found only in specialized clinics, university vision labs, or occupational testing centers. Most people will never need one unless a screening test flags a deficiency that requires precise classification.
Online Tests: Useful but Limited
Digital color vision tests have become popular for quick self-screening. A 2025 study comparing one validated web-based tool against the standard Ishihara test in 330 patients found an overall concordance rate of about 88%, with 96% sensitivity and 99% specificity for detecting color vision deficiency. That makes well-designed online tests a reasonable first check, but not a replacement for clinical evaluation.
The main limitation is your screen. Monitor calibration, brightness settings, ambient lighting, and display age all affect how colors appear. Two people taking the same test on different laptops may see meaningfully different colors. If an online test suggests you have a deficiency, follow up with an in-person evaluation using physical test materials under controlled lighting.
Testing Children
Color vision deficiency often goes unnoticed in children because they adapt to how they see without knowing it differs from anyone else. Screening guidelines from the American Academy of Pediatrics recommend instrument-based vision screening starting between 12 months and 3 years at annual well-child visits. Color vision screening specifically should be considered for preschool and school-aged children, especially boys, given the much higher prevalence in males.
Young children who can’t yet read numbers can be tested using tracing versions of pseudoisochromatic plates, where they follow a colored path with their finger instead of identifying a digit. For children who are very young or uncooperative with standard tests, photoscreening and handheld autorefraction can detect conditions that affect vision development, though these focus more on structural eye problems than color discrimination specifically.
Acquired Color Vision Loss
Not all color blindness is inherited. A number of medical conditions can damage the retina or the neural pathways involved in color processing, leading to color vision changes that develop later in life. These include diabetes, macular degeneration, glaucoma, multiple sclerosis, Parkinson’s disease, Alzheimer’s disease, sickle cell anemia, chronic alcoholism, and leukemia. Certain medications, including hydroxychloroquine (used for rheumatoid arthritis and lupus), can also affect color perception.
Acquired color deficiency differs from inherited in a few key ways. It can affect one eye more than the other, it often involves blue-yellow discrimination rather than red-green, and it can worsen or improve depending on the underlying cause. If you notice colors looking different than they used to, particularly if the change seems to affect one eye, that warrants an eye exam that includes color vision testing. The Farnsworth-Munsell 100 hue test is especially useful here because it can track subtle changes over time.
Career and Licensing Requirements
Certain professions require verified normal color vision. Aviation is one of the most structured. As of January 2025, the FAA requires first-time pilot applicants to pass an approved computer-based color vision screening test. Applicants who fail every available approved test receive a third-class medical certificate restricted to daytime visual flight rules only. Upgrading to a first- or second-class certificate after a failure requires an appeal to the Federal Air Surgeon. Pilots who have previously passed an FAA-approved color vision test do not need to retest, as the policy shifted to one-time screening for inherited deficiency, with separate protocols for acquired changes due to medical conditions or medications.
Maritime officers, electricians, railway workers, and law enforcement officers also face color vision requirements, though the specific tests and thresholds vary by country and employer. If you’re entering a field where color discrimination matters, find out which specific test is required before your exam. Passing one test does not guarantee passing another, since each test targets slightly different aspects of color perception.