Colour Blindness

Colour Vision Deficiency or ‘colour blindness’ is a condition where you may not be able to see certain colours accurately. You may not be able to tell the difference between reds and greens for example. Here’s a very brief breakdown of how this happens.

When light comes through the lens of the eye it hits light sensitive elements at the back of the eye called cones. Cones break down the light entering the eye into 3 different colours, either red, blue or green. Each eye has a set of cones sensitive to red light, a set sensitive to blue light and a set sensitive to green light. The cones then send their individual red, green or blue colour data to the brain where it is reassembled to produce the actual colours of the scene you are looking at. If you did art or physics at school you may remember that all colours are made up of a combination of red, green and blue so mixing different levels of red, green and blue will make up any colour on the visible spectrum. 

If all of your cones are working properly your brain will receive the correct colour information to be able to accurately reproduce all the colours of the scene you are looking at.  If this is the case you are said to have trichromatic or ‘normal’ vision.  If any of the 3 types of cones are not working properly your brain will not get all the information needed to accurately reproduce the colours you are looking at and you will consequently perceive some colours differently.  In this case you are said to have a colour vision deficiency or colour blindness.  The most common colour deficiencies are with reds and greens appearing to be the same colour.

This diagram below will give us an idea of what colours might look like to people with both ‘normal’ vision and various colour deficiencies. The ‘Normal Vision’ colour wheel shows the full colour spectrum seen by people with normal (trichromatic) vision and then the colour wheels of various colour deficiency conditions show what people with these conditions would actually see instead.

Trichromacy or normal vision is where all the colours of the visible spectrum are perceived.

Tritanopia leaves you unable to tell the difference between blue and green, purple and red, and yellow and pink. It also makes colours look less bright.

Achromatopsia is a condition characterized by a partial or total absence of colour vision. People with complete achromatopsia cannot perceive any colours at all and can see only black, white and shades of grey.

Finally Deuteranopia, the most common colour deficiency, is a type of red-green colour blindness in which the green cones in the eye detect too much red light and not enough green light. As a result red, yellow, green, and brown can appear similar, especially in low light.  In laymans terms, it makes reds look green.

Image shows the colour wheel of someone with normal or trichromatic vision and the vison wheels of people with different colour deficiencies

Since deuteranopia is the most common colour deficiency we will use it as an example of how it might affect someone accessing colour content on a website.  In the diagram above we have a graphic which, to someone with full colour vision, will appear to be a green letter on a red background.  To a person with deuteranopia who cannot perceive reds fully this will appear to be a green letter on a green background.

Image shows 2 squares. One with a green R on a red background and one completely green

General advice for accessibility for people with colour blindness is to be careful about how you use colour combinations and try not to use colour alone in your graphics to get messages across.  In the example below we can see that putting text into the graphics will help someone with colour blindness understand their meaning fully as opposed to just using the plain coloured graphics.

There are a variety of online resources which you can use to check your online graphics and images to ensure they are accessible  for people with colour blindness.  I have listed some at the end of this course.