Edit to add: check the comments section for further guidance from readers!
According to the Colour Blind Awareness organisation, colour blindness (colour vision deficiency) affects around 8% of men and 0.5% of women, or around 4.5% of people around the world.
The most common type of colour blindness is deuteranomaly, which means the eyes are less sensitive to green light. People with this condition can’t easily distinguish between reds and greens, as shown in the image below.
Scientific figures and colour blindness
Although many journals recommend that authors make their figures accessible to people with colour blindness, problematic images are still common. In the life sciences, we often see red-green fluorescence microscopy images, which can appear brown to people with deuteranomaly. By making your images as accessible as possible, you will help the 4.5% of editors, reviewers and eventual readers with colour blindness to understand your important discoveries!
You can read more about the different types of colour blindness here, but it is important to note that reds and greens are not the only problematic colour combination. Some people can’t distinguish blacks and reds, bright greens and yellows, greens and oranges, reds and browns, blues and purples, or greens and browns, among other colour groups.
For this reason, we recommend testing how they might look for people with different types of colour blindness. Check out this colour blindness simulator (the one used for the above image), which allows you to see how people with colour blindness would see your image. The calculations are all done by your own computer, so no-one else can see them.
Making figures accessible to people with colour blindness
To counteract the problem of colour blindness, consider using different symbols, patterns or shades of a single colour to represent different samples in graphs and line drawings. Alternatively, you can find a palette of colours that are unambiguous to people with colour blindness and normal vision on the following excellent resource from the University of Tokyo: http://jfly.iam.u-tokyo.ac.jp/color/index.html
For microscopy, many people recommend using green and magenta (purple) rather than green and red. Magenta and green can be distinguished by people with most types of colour blindness. In fluorescence images, the overlaid magenta and green colours produce a double-positive region in white. Red-green images can easily be converted to magenta-green in image manipulation software by copying the red channel to the blue channel, as is also described in detail in the University of Tokyo resource: http://jfly.iam.u-tokyo.ac.jp/color/index.html#convert
Don’t risk confusing or alienating your readers. Apply these simple checks and solutions to make your figures more accessible and maximise the impact of your work.