They're beautiful, but a small tangent... the latest macOS has a bug that repeatedly reverts the wallpaper to the default whenever I unplug/plug my external monitor and it's driving me crazy.
I noticed this a few days ago when moving a space from one monitor to the other and seeing its desktop picture change. Moving the space back to the original monitor restored the desktop picture.
Not sure why it’s this way, kind of an odd choice.
Because of how the new system work, I wouldn't be surprised the bug is limited to those solid color backgrounds (which are handled quite differently for historic reasons, and it could very well be why you get the error).
The new wallpaper system is a bit of a mess, and solid colors have always been treated weirdly in their preferences.