Data visualizations using rainbow color scales are ubiquitous in many fields of science, representing everything from ocean temperatures to brain activity to Martian topography. But cartographers have argued for decades that the “Roy G. Biv” scale makes maps and other figures difficult to interpret, sometimes to the point of being misleading. And for color blind people, they are totally unintelligible.
Today, scientists at a US Department of Energy lab have developed a color scale that is mathematically optimized to be accurate for both people with color blindness and people with normal vision. The scale was described Wednesday in a new study in PLOS ONE. “People like to use the rainbow because it catches the eye,” says lead author Jamie Nuñez, chemical and biological data analyst at the Pacific Northwest National Laboratory (PNNL). “But once the eye gets there and people try to figure out what’s really going on inside the picture, that’s where it kind of breaks down.”
Ditching that multi-colored ladder can even save lives. Researchers at Harvard University found that when traditional 3D computer models of arteries in rainbow colors were replaced by 2D models using a color scale from red to black (pdf ), the accuracy of physicians in diagnosing heart disease increased from 39% to 91. percent.
There are several reasons why the rainbow color scale is problematic. Perhaps the most fundamental problem is that the relationship between colors is not intuitive. “The problem with the rainbow is that you don’t see it perceptually as ordered,” says Colin Ware, an expert in human perception and data visualization at the University of New Hampshire who was not involved. in the study. “If you give people the colors red, blue, green, and yellow, they won’t know what order to put them in. “
Another problem is that the brain naturally interprets differences in brightness, or luminance, as representing depth, with the colors most vivid at the top. This is why gray scales are good for representing topography (pdf) and shapes: people see black as the lowest part of the scale, going increasingly lighter gray with white at the top. . But among the colors of the rainbow, yellow has the highest luminance and will often appear to represent peaks on a map, although it is usually supposed to be somewhere in the middle of the scale, with blue below and red above. Nuñez and Ryan Renslow, PNNL chemical engineer, tackled this problem with their new scale, which they call cividis, using just two colors with a clear luminosity hierarchy: blue and yellow. Just like with a grayscale, people perceive the brightest yellows as peaks and the darker blues as downs. But viewers can perceive a greater level of detail with colors instead of shades of gray.
To create the new color scale, scientists built a software tool based on an advanced mathematical model of human vision. The tool takes an existing color scale, simulates what it looks like for people with red-green color blindness (the most common form, affecting 7-10 percent of men and a tiny fraction of women), and then the adjusts so that both colors and brightness vary at a constant rate across the scale. This ensures that the scale accurately represents the underlying data. “I think it has a lot of very good properties,” says climatologist Ed Hawkins of the University of Reading in England, who has been pushing for scientists in his field and beyond to abandon the rainbow. sky. Hawkins was not involved in the development of the new scale.
By mathematically optimizing their scale to be perceptually consistent in people with color blindness and those with normal color vision, Nuñez and Renslow avoided another major pitfall of the rainbow color scheme: with cividis, the perceived change in hue and luminance corresponds to the actual change in the data. But with the rainbow, people see the transitions between some colors as gradual and others as abrupt, which can make it seem like there are sharp limits in the data where it isn’t. there is not any. This effect can even lead scientists to misinterpret their own data, Hawkins says. A particularly striking example is a 2006 study that identified the boundaries between colors on a rainbow map of atmospheric ozone data as prominent “fronts”. But those limitations disappeared when the data was plotted using a different color scale.
Scientists are increasingly aware of the dangers of the Rainbow Scale, at least in part thanks to calls from cartographers and scientists like Hawkins. But scientific journals, conferences (pdf) and press releases are still littered with rainbows. “People love colorful maps,” says Kenneth Field, cartographer at mapping software company Esri and outspoken critic of the Rainbow. Although a card with a single color that varies from light to dark to show increasing values would in many cases be much lighter and easier to read, some people may find it dull compared to its full-spectrum cousin, Field says. .
Some scientists may prefer the rainbow because it can convey more details about their data. Humans can only see about 30 different shades of gray, but millions of different colors. But because the rainbow will perceptually distort those details, Field says, any advantage is likely outweighed by the disadvantages for most visualizations. “Rainbows cause more problems than they solve.”
Yet this has long been the default color scheme for most data visualization software available, and scientists have grown used to it. Hawkins and other scientists have made inroads with software companies, convincing some to change their default settings to better color schemes. Nuñez and Renslow successfully lobbied for Cividis to be added to the color scale libraries of a dozen software programs. And they’ve created an open-source software tool that can help scientists create their own color scales optimized for their particular datasets and visualization needs.
Hawkins hopes more scientists start making informed decisions about which color scale to use, but he thinks it may take a push to precipitate real change. “There may be a role at some level for scientific journals to step in and say they won’t accept papers that use rainbow color scales.”
In the meantime, with climate change and extreme weather increasingly in the news, people are unlikely to escape the rainbow map.