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Not Just Black and White: Colour Blindness Explained

Dr Imran

3 min read

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While the name suggests a complete inability to see colours, this is merely one type of colour blindness (achromatopsia); the more common phenomenon is known as ‘Colour Vision Deficiency (CVD)’, in which affected individuals have difficulty identifying the shades and tones of red, green, or blue either partially or completely.

How Does Your Eye See Colors?

They eye can be thought of as a camera, with the front part being the lens, which is responsible for focusing images inside the back of the eye. This area in the back is called the retina. It is enclosed in special nerve cells that have pigments which react to light. They are the following:

Cones: These control the color vision of the eye. There are three types of cone cells that have several kinds of pigments. They all react to different wavelengths of light, specifically short, medium and higher.

Rods: These only have one type of pigment which reacts the same way to all light wavelengths. They have no relation to color vision, but they are very sensitive to light and help us to see in the dark, such as at night.

How It Happens:

Doctors says, the average human possesses 6-7 million cones (the light receptors in the retina that help with colour identification), all of which are grouped into three categories corresponding to either red, green, or blue. The absence or weakness of one or more of these cones makes an individual ‘colour blind’. For instance, someone with abnormal or non-functioning ‘red-detecting cones’ will have difficulty identifying red-coloured objects or those with the colour red in them.

Other Causes:

While the main reason for CVD is a hereditary genetic defect (in which case individuals are usually colour blind from birth), other external factors may also trigger the condition:

  • Injury or Damage to the visual processing areas of the brain.
  • Parkinson’s Disease damages photosensitive nerve cells in the retina responsible for vision processing.
  • Cataracts that cause clouding over the lens can ‘mute’ colours by obstructing the light that enters the eyes. However, the visual spectrum is restored after cataract removal.
  • Antiepileptic Drug Tiagabine may temporarily reduce colour vision in some patients during the course of treatment.
  • Leber’s Hereditary Optic Neuropathy (LHON) may cause red-green CVD in certain individuals as it causes degeneration of retinal cells.
  • Kallman’s Syndrome, a pituitary gland disorder, may also present CVD as a symptom.
  • Similarly, CVD may also be a symptom in certain patients with diabetes or multiple sclerosis.


1- Protan (Red) Deficiency:

Those with protanomaly have abnormal red cones, and hence see muted, greener versions of reds, oranges, and yellows.

Individuals with protanopia only see yellow instead of oranges, and greens, and see reds as black due to dysfunctional red cones.

2- Deutran (Green) Deficiency:

Deuteranomalous: individuals see redder versions of yellows and greens and cannot differentiate between violet and blue due to limited functioning green cones.

Conversely, those with Deuteranopia see greens as black or beige and reds as brownish-yellow.

Red-Green deficiencies are the most common forms of CVD and affect more males than females.

3- Tritan (Blue) Deficiency:

An abnormality in the blue cones (tritanomaly) makes blues appear greener, while making differentiation between yellow, red, and pink difficult.

Individuals with no blue cones (tritanopia, or blue-yellow deficiency) see blue as green and yellow as light grey or violet. However, blue-yellow CVD is highly uncommon.


Of the numerous testing methods developed, three of the most common tests are:

1- The Ishihara Test:

The most widely-employed deficiency detection test, the Ishihara Test comprises of 38 plates with one or two-digit numbers hidden within a pattern of irregular colored dots. Test-takers must identify the hidden numbers, on the basis of which color deficiencies are detected. However, for children between 3 and 6 who cannot yet identify numbers, plates with simple shapes and pictures are used instead.

2- Cambridge Colour Test:

Users are asked to locate C-shapes hidden within multi-coloured disks similar to the Ishihara test, except on computer screens. Furthermore, the users must identify the correct orientation of the letter from one of four given orientations in each disk.

3- Farnsworth-Munsell 100 Hue Test:

This test is employed in graphic design, photography, and electricity companies, among others, where accurate colour perception in employees is a necessity. It comprises specific-colored blocks given to the test-takers, who must then arrange a certain number of pegs or blocks in different shades/hues of the given colour in order.

Note: All of the above tests can only detect red-green deficiencies.


While there is no cure for CVD, most affected individuals can learn to identify non-visible colours by committing them to memory, like memorizing the order of traffic lights in red-green deficient individuals, and using specialized apps that aid in colour detection for day-to-day tasks. Some red-green deficient individuals may also use a specialized lens to enhance colour perception, albeit only outdoors under bright light.

Since severe CVD can cause hindrances in daily-life activities, consult your doctor as soon as you start experiencing difficulties in colour perception or if you have a family history of the condition. You can also book an appointment with a top optometrist or ophthalmologist in Lahore, Karachi and Islamabad through oladoc.com, or call our helpline at 042-3890-0939 for assistance to find the RIGHT doctor for your ocular concerns.

Disclaimer: The contents of this article are intended to raise awareness about common health issues and should not be viewed as sound medical advice for your specific condition. You should always consult with a licensed medical practitioner prior to following any suggestions outlined in this article or adopting any treatment protocol based on the contents of this article.

Dr Imran - Author Dr. Imran Akram Sahaf is a Eye Surgeon practicing in Lahore. Dr. Imran Akram Sahaf has the following degrees: MBBS, MCPS (Ophthalmology), MRCP and has 41 years of experience. You can book an appointment with Dr. Imran Akram Sahaf by calling us or using the 'book appointment' button. He loves to write on healthcare to raise awareness about general healthcare issues in Pakistan and advise patients on healthy living.
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