How The Rainbow Color Map Misleads

Colors are perhaps the visual property that people most often misuse in visualization without being aware of it. Variations of the rainbow colormap are very popular, and at the same time the most problematic and misleading.

The rainbow color map is based on the colors in the light spectrum, and is sometimes done correctly, sometimes the colors are in the wrong order. Quick, name the colors in the rainbow in order! See, that’s part of the problem. Even if they were used consistently, nobody would know the right sequence anyway. Here is an image to jog your memory, courtesy of Wikipedia.

Now take a look at this map from a paper on water resources published in the Journal of the American Water Resources Association, which I found on Cliff Mass’s fantastic weather blog. It describes the amount of evapotranspiration (loss of rain water through evaporation) by county for the 48 contiguous U.S. states.

Do you see the how the country is divided down the middle? The Eastern half seems to be all dark green and blue, while the Western half is all light greens, yellow and orange. Surely, there is a huge difference between the two.

But let’s take a closer look at the legend.

As it turns out, the values change smoothly, but the colors do not. There are two problems here: abrupt changes in luminance (perceived brightness of a color) as well as switching between different hues.

The combination of smoothly [...]