How to make plots accessible for the color blind

I am an anomalous trichromat. Anomalous trichromacy is a type of color vision deficiency where one of the three cone photoreceptor pigments (which are necessary for color vision) in the retina has an altered sensitivity range. Humans normally have three types of cone cells, each sensitive to different wavelengths of light: long (L), medium (M), and short (S) wavelengths, corresponding broadly to red, green, and blue light, respectively.

In people with healthy color vision, these three pigments have peak sensitivities at different wavelengths, which allows for the perception of a wide range of colors. When one has anomalous trichromacy, however, the spectrum of light to which one of the cone pigments is sensitive is shifted towards another. This shift results in an overlap in the sensitivity of the cones, causing difficulties in differentiating between certain colors.

The altered sensitivity does not mean that individuals with anomalous trichromacy cannot see colors at all, but rather that they see them differently and often less vividly or with less distinction between certain shades than those with typical trichromatic vision. This condition is often genetic and present from birth. It can be diagnosed with color vision tests.

When giving presentations or making plots for publications, it is easy to accidentally use colors that are hard or impossible for people like me to distinguish. Around eight percent of men and a much lower percentage of women are affected by conditions like mine and there is a very high chance that at least someone in your audience is. Therefore, you should only rely on color to communicate information when it’s necessary and choose colors that are easily distinguishable.

Here are some strategies to mitigate the problem:

1: avoiding colors entirely

• use symbols like crosses, circles, triangles, and squares instead of colors to mark data points on a plot
• use hatching instead of colors to distinguish bars on histograms and bar plots

2: using color palettes that were developed to help people like me

• Paul Tol’s website and his write-up are great resources for that
• This post by Joachim Goedhart is also helpful
• Good plotting tools (e.g., ggplot2, MATLAB, etc) have convenient ways to use the colors of your choosing.

3: checking what a plot / presentation looks like to people with different color deficiencies

some simulation tools (I can’t guarantee they really simulate the correct thing):

• color oracle
• pilestone is a company selling glasses that help “fix” color impairments. I am not affiliated with them. I don’t know if it works. Here is their simulation tool
• your friend who is color blind might check your plots if you bribe him with coffee ;)