We use colour in many aspects of our lives to help draw attention to things or to distinguish one item from another. Some people do not have typical colour vision and as a result their perception of colour is altered so that they have a reduced range of colours they can uniquely identify within the visual spectrum.
Colour vision deficiency (CVD) is more commonly known by some under the name colour blindness. CVD is more mainly found among males since the more frequent types are due to a defect on the X-chromosome. The different types of CVD are: protanopia (1.01% males and 0.02% females), deuteranopia (1.27% males and 0.01% females), protanomaly (1.08% males and 0.03% females), deuteranomaly (4.63% males and 0.36% females). Tritanopia and tritanomaly are much more rare (between 1:13,000 and 1:65,000), and affect males and females equally.
It is important to consider people with CVD when designing websites, since in the UK alone there would be at least 2.5 million people who might have trouble using the website and this number is much larger on a world wide scale. It was mentioned before that colour can draw attention to something or distinguish one item from another. To give an example that relates to websites, it might be that text has been given a different colour property to stand out, yet for someone with a specific type of CVD it might disappear against the background due to a lack of contrast. Therefore, it is important to keep this in mind. Similarly, if the user had to pick something, such as the confirmation button rather than cancel button when making a purchase, then it would be easier if the text had sufficient contrast against the button background colour.
The tool on this website should make it much easier to see how colours are perceived with three different types of CVD and it will make it clear whether a colour combination is going to cause difficulties for a person.
Some helpful information about CVD can be found on this NHS page and also at colourblindawareness.org. Statistical information gathered from this source.