Using a Perceptually Valid Color Space
Although almost everyone can discriminate hues, not everyone can discriminate the same hues or with the same fidelity. So it’s important for the color scale to also vary in lightness. You can verify the lightness looks ok by looking at how the scale would appear to someone who cannot perceive color:
What’s happening here? You’re probably familiar with RGB — the triple of red-green-blue color components that represents colors shown on computer monitors. Unfortunately math in “RGB space” isn’s perceptually valid. Dividing the green component in half doesn’t make a color half as green. Adding 50 to all of components makes the color brighter by uneven amounts depending on which color you start with. That’s because the sensitivity of human color perception is uneven across the gamut of colors we can see, and the definition of RGB used by computer monitors doesn’t follow human perceptual sensitivity.
Color professionals have come up with alternative measurement systems for colors so that taking consistent arithmetic steps has a perceptually consistent effect. One such system replaces RGB with CIE 1976 L*, u*, v* (CIE LUV) color space, and RGB’s cylindrical equivalent HSL with CIELCH. This page uses CIELCH to interpolate colors evenly across their lightness (L), chroma (C, similar to saturation), and hue (H).
Color math is performed with d3-color. Some of the suggested end colors are inspired by Color Brewer’s end colors.