Colored Shadow Penumbra

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.

What's the physical basis for this effect? Does it happen in reality or is it a style choice?

causes the external display I have plugged into my MBP (clamshell mode) to flicker quite badly

Cool!

FYI the arrows on both photos only actually control the top photo.