Your Guide to Colour Blindness

Colour blindness is a commonly used term to describe a condition that makes it difficult to discern the difference between certain colours and shades. 

The correct term for colour blindness is colour vision deficiency. In true colour blindness, no colour can be seen at all - as this is very rare, colour vision deficiency is a more appropriate description.

Colour vision deficiency makes it difficult to tell the difference between certain colours. Often, a group of colours will all appear to be the same shade.

There are several different types of colour vision deficiency, some more common than others. The actual severity of the condition varies depending on the person.

Rods and Cones

The eye’s retina contains cells called rods and cones, which respond to light. After the rods and cones respond to the incoming light, neural cells process the signal.

The cone cells can be divided into three different types. These are the short-wavelength (S), middle-wavelength (M), and long-wavelength (L) cones.

S-cones are most sensitive to light in the violet part of the spectrum. M-cones are most sensitive to light in the green part, and L-cones are most sensitive to light in the green-yellow part.

The rods and cones are laid out in the retinal wall as below:

Red-Green Colour Vision Deficiencies

US Flag colour blindThe most common types of colour blindness are in the red-green colour vision deficiency classification. People with these deficiencies confuse colours that lie within the red-green colour spectrum. The condition affects 1 in 12 men, but only 1 in 200 women have a red-green colour vision deficiency.

This difference between the sexes is because the genes for the red and green colour receptors are on the X chromosome. Females have two X chromosomes, while males have only one. Red-green colour blindness only occurs in Females with both X chromosomes defective, while males need just their single chromosome to be affected.

Red-green colour vision deficiencies occur due to problems with either the L-cones or M-cones. A mild colour deficiency occurs when one or more of the three types of cones are weak or faulty, which results in Protanomaly or Deuteranomaly.

A severe colour deficiency occurs when a type of cone is missing altogether. This results in Protanopia or Deuteranopia.

The four types of red-green colour vision deficiencies are as follows:

Protanopia is a deficiency of L-cones. You’re likely to confuse reds, yellows and greens. Reds are likely to appear darker than normal.
Protanomaly is a less severe deficiency of L-cones. You’re likely to confuse shades of the same colour from the red-green-yellow spectrum.
Deuteranopia is a deficiency of M-cones. Similar to Protanopia, you’re likely to confuse reds, yellows and greens. The hue tends to shift towards the red end of the spectrum.
Deuteranomaly is a less severe deficiency of M-cones. Like Protanomaly, you’re likely to confuse shades of the same colour from the red-green-yellow spectrum.

Protanopes are likely to confuse:
1. Black with many shades of red
2. Dark brown with dark green, dark orange and dark red
3. Some blues with some reds, purples and dark pinks
4. Mid-greens with some oranges

Deuteranopes are likely to confuse:
1. Mid-reds with mid-greens
2. Blue-greens with grey and mid-pinks
3. Bright greens with yellows
4. Pale pinks with light grey
5. Mid-reds with mid-brown
6. Light blues with lilac

Blue-Yellow Colour Vision Deficiency

Blue-yellow colour vision deficiencies are much rarer than their red-green counterparts, occurring in less than 1 in 10,000 people. There’s no difference between the two sexes in its occurrence, as it is not associated with the sex chromosomes.

Blue-yellow colour deficiency is usually associated with deficiencies in the S-cones. People with this condition have trouble telling greens and blues apart.

The below chart demonstrates the altered colour spectrums for people with colour vision deficiencies, and how common each type is:

Colour blind chart

Are You Colour Blind?

Most people realise they have a colour vision deficiency naturally, after being unable to identify certain colours correctly. However, in some (usually mild) cases, colour blindness is only confirmed after special testing, as colour vision deficiency isn’t always covered in standard eye examinations.

Ishihara Plate 15

The most common way to test for colour blindness is using a series of Ishihara Colour Vision charts. Using these charts, it is possible to test yourself for red-green colour deficiencies.

The full Ishihara test consists of 38 charts, though a colour vision deficiency is usually evident after viewing just a few charts.
In the example to the left, those with normal colour vision will see a ‘15’. Those with a red-green colour vision deficiency will see a ‘17’.

Ishihara plates are only used for initial testing, and do not provide a formal diagnosis on their own. If you are ever unsure about whether you are colour blind or not, or if you have any questions, seek professional help from an optician.

Living with Colour Blindness

While colour vision deficiency doesn’t cause any long-term health problems, it can have an impact on many different aspects of life. Jobs that require discerning between colours may prove to be difficult, or even impossible.

Having a red-green colour vision deficiency can make it hard to spot blood in body fluids, which could otherwise be an early warning for a medical condition. Children with a colour vision deficiency may struggle in school with certain tasks, particularly where colour coding is involved.

Studies have also shown that colour blindness can be beneficial in certain situations. For example, some colour vision deficiencies have been shown to make it easier when perceiving camouflage patterns.

It is not currently possible to replace or repair cells in the retina. This means that there is no known cure for colour blindness. However, other methods have been proposed to help colour deficients distinguish colours more easily. Wearables such as the ‘Oxy-Iso’ glasses are able to enhance the contrast between certain colours, allowing those with red-green colour vision deficiencies to pass the Ishihara test.

Glasses like the Oxy-Iso aren’t a cure for colour blindness, and are only helpful in certain circumstances. Light intensity is reduced, while the ability to perceive yellows and blues is diminished, making them unsuitable for driving.