People with colour blindness have a difficulty differentiating between certain colour hues (there is a lot of very good information about colour blindness at www.colourblindawareness.org). This doesn’t mean that they cannot see colour at all, just that there are certain colours between which it is difficult to distinguish. The most common forms of colour blindness (deuteranope and protanope) lead to difficulties distinguishing between red and green, while a less common form (tritanope) leads to difficulties distinguishing between blue and yellow.
Colour blindness affects about 10% of men and about 1% of women, so it is something that we should take seriously when designing data visualizations. There are three easy things we can do to mitigate the effect of colour blindness on people’s ability to read a data visualisation. The first is, if we really must use a colour scheme that is not colour blindness safe, is to use a redundant visual encoding such as shape in our visualisations. The images below show a version of the scatter plot above where bad instances are shown as red circles and good ones are shown as green stars. Someone who cannot distinguish between the colour differences, should still be able to distinguish between the shape differences (although the shape differences are not as effective when points start to get crowded and overlap).