I'll be giving a job talk in a month or so and I have been meaning to go through my standard slides and make sure that I am using color-blind friendly palettes.  For the last year or so everything that I do with R, I tend to do using the viridis package which I know is supposed to be fairly good.  However, for items that I make in powerpoint I often use the website ColorBrewer, and in the past, I may not have paid that much attention to this issue.

I have some pretty intricate animated slides and my first question was whether it was even worth messing with some of these. Perhaps it was already good enough.  To make this decision I needed to know two things. 1) What is the incidence rate of color blindness by type in the US? 2) What will these slides look like to a person who is color-blind?

So first the incidence rates.  This varies a lot by type and population but a good rule of thumb is around eight percent of men and 0.5 percent of women have some form of dichromatic vision.  The other types of color blindness are rare enough that I'm going to focus on dichromats for now. For normal trichromatic vision your receiving information on the intensity of light by your rods as well as three types of cones that are most responsive to either short, medium, or long wavelength light.  Commonly people refer to these as blue, green, and red cones respectively despite the fact that they don't really match up to well to those colors.  The information from these cones are what allows us to determine the wavelength (color) of objects we see.  In contrast, dichromats have only two of these three types of cones leading to greater difficulty in distinguishing various shades.  I would never consciously give a talk that was I knew was confusing to 4-8% of my audience¹.  So the incidence rate is definitely high enough that I can't use that as an excuse.  My slides need to be color-blind friendly!

Next question how bad are my slides as is?  It ends up there are some great resources on the web that let you get some idea of what your slides will look like to people with different types of color blindness.  I particularly like the site CoBliS .  With this site, you upload a PNG or JPG file and then choose the type of color blindness to simulate.  I do a lot of work with phylogenies and I have often used phytools contmap function to illustrate phenotypic evolution across a phylogeny.  The default for this function uses the rainbow palette to produce a range of colors for the trait being analyzed:


In general, people really like these.  Personally, I love them they are quite striking.  What does this look like to someone who is color-blind?  I checked out several different types of color blindness that can be simulated with websites like CoBliS.  The two most common types of color blindness in the population are protanopia and deuteranopia.  I found that in several of the images that I checked the specific colors that were challenging differed a bit but in general, a bad palette was bad and a good palette was good.  For that reason, the examples that I show below are all simulated as visualized by someone with protanopia color vision.

Obviously, this simply isn't going to work.  In particular, we see that the important distinction between green and yellow is simply missing for viewers with one of the most common types of color blindness.  We need to find a palette that is at least marginally better. Let's try it again with viridis palette.