I love the 'oklab' color space. This space is approximately perceptually uniform, meaning that --- unlike the common RGB space --- distance in oklab space is meant to correspond to perceived color differences.
For visualiztaion its often useful to have uniform, cyclic color maps. Using oklab, we can find a rainbow colormap --- which varies only hue --- by looking for the largest inscribed circle orthogonal to (and centered on) the luminance axis.
Amazingly, the biggest inscribed circle along the luminance axis in the OKlab gamut occurs at ℓ ≈ 75%. The author of oklab, Björn Ottosson, confirmed that this was serendipitous
That isoluminance circle has a radius of about 0.1275. The biggest circle I could find in the oklab gamut has almost twice the radius (0.2338). Trades some of that rainbow hue change for luminance variation. It doesn't have as pretty colors.
In contrast, consider (one of) the largest circles in RGB space.
This is just one of 4 largest circles. Here are the others.
