At the end the series The rainbow is dead…long live the rainbow! I introduced CubicYF, a rainbow-like color palette with a slightly compressive, monotonic increasing lightness profile. The red color is missing from the palette because green and red at people with deuteranopia, the most common type of color vision deficiency, confuse red and green, so I took one out. An alternative solution to taking the red out is to have a yellow-like color with lower lightness followed by a red-like color of higher lightness, like in the LinearL palette described The rainbow is dead…long live the rainbow! – Part 5. A third solution, which is a compromise in perceptual terms, is CubicL (sometimes I called it cube1), the color palette I used with the gravity data from my degree thesis in geology in the series Visualization tips for geoscientists. An example map from the series in which I used CubicL is in Figure 1 below:
To generate CubicL I used a compressive function for the increase in lightness very much like the one for CubicYF, except in this case I allowed lightness to decrease from about 90 in the yellow portion of the colormap to about 80 to get to the red. As a result, there is an inversion in the lightness trend, indicated by the arrow in the bottom row in Figure 2. However, I believe this is an acceptable compromise, and one that is pleasing for people accustomed to rainbow (it has red), because the inversion is smooth, with a gentle downward-facing concavity. I also believe this colormap is still a perceptual improvement over the rainbow or spectrum so commonly used in seismic visualization, which typically have at least 3 sharp lightness inversions (indicated by arrows in the top row of Figure 2 below).
CubicL is one of the colormaps available in my Matlab File Exchange function Perceptually Improved Colormaps.