Colourblindness and graphics

About one in ten males (including myself) have some degree of red-green colour blindness (and 0.5% of females), so you should bear that in mind when you are making graphics to display to others. Red-green colour blindness comes in varying degrees – it isn’t necessarily that red and green cannot be distinguished, but that are clearly different to people with normal colour vision may be hard (or impossible) to discriminate for others. Here are some hints.

Thin lines are difficult. Thin lines don’t activate many colour sensors (cones) in the retina, so it may be difficult to discriminate the colours of thin lines. On the graph to the right I am hard pressed to tell which line is which colour. In fact, I am not certain these lines are coloured – they could be grey.

Making the lines thicker aids clarity and makes the colour MUCH easier to discriminate. In this version of the graph I can work out which line is red and which is green without too much effort, at least at the size this shows on screen in this page. If one is sitting at the back of a theatre looking at a tiny version on a screen … then we are back to the too few cones activated problem.

Using solid symbols adds extra colour area to activate more cones. This helps. But for those on the weaker end of the colour blindness spectrum it still makes for difficulties.

Here we have a secondary means of discriminating the lines – hollow vs solid symbol and dashed vs continuous line. Now the lines are clearly distinct, even for those who are totally colour blind.

Alternatively, to accommodate red-green colour-blindness, use a colour palette which is friendly. The blue line is clearly distinct from the red ??? or is it green or grey??? Whatever, the two lines are distinctly different because I can see blue well.

Another approach is to use different degrees of brightness, as well as different colours, for the lines and symbols. One can see differences in shade clearly on this version even if you cannot discriminate any colours.

There is a lot of information on the web that explains the issues of colour blindness, and many tools to help you select colours (and other strategies) that will make your data presentation much clearer for colour blind people (and, in reality, also easier for people with normal colour vision). Here is a selection of links that look useful … or do your own search (eg