Wednesday, June 26, 2019

Our understanding of rainbow color schemes remains incomplete

Rainbow color schemes are the 3D pie charts of the sciences: they are are everywhere even if they are considered dubiously useful by visualization professionals. Last year Betsy Mason wrote a nice summary about the shortcomings of rainbow palettes for Scientific American magazine: rainbows don't just make you see categorical boundaries in otherwise continuous data, but also,
the relationship among the colors is not intuitive. “The problem with the rainbow is that you don’t perceptually see it as ordered,” says Colin Ware, a human perception and data visualization expert 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 will not know which order to put them in.” Another problem is the brain naturally interprets differences in brightness, or luminance, as representing depth, with the brightest colors at the peak.
This said, we may be just scratching the surface A recent paper by Sam Quinan, Lace PadillaSarah Creem-Regehr, and Miriah Meyer explores whether the illusory discretization that rainbow color schemes elicit is the same for all readers, for all data sets, and in all variations of these spectral palettes:

The answer is that, yes, rainbow color palettes are tricky, but (a) there are differences between them, (b) the nature of the data represented affects the way inexistent boundaries on continuous scales are perceived, (c) luminance doesn't seem to be the only factor that affects discretization; chroma and hue may also play a role.

The paper ends by wondering why, despite their repeatedly proven shortcomings, rainbow schemes are still so popular—convention, familiarity, and attractiveness, but also the fact that scientists find them useful sometimes,—and by suggesting paths for further research as, quoting the authors, “the visualization community’s current understanding of how rainbow color maps are perceived and used remains incomplete.”