This page helped me understand what they’re going for: https://patapom.com/blog/Lighting/Colored%20Penumbra/Colored....

The way I’m intuiting it: some things will “glow” when strongly illuminated, and the glow is more colored than the reflected light, so if the illumination has a hard edge then the penumbra can end up saturated by the more strongly illuminated part’s glow.

OP’s rendition isn’t quite landing for me, though, and I’m not enough of an artist to be sure why. Maybe it’s just that saturation is cranked way up for the demo, but it might also be that it shouldn’t occur on rock, or that color seems sometimes to not react to a change in which material is glowing.