Color profiles and night light are only applied to primary monitor
I have two monitors, but when I change the color profile or enable night light mode, it only affects the primary one.
For debugging, I patched Mutter to log all calls to "meta_monitor_manager_handle_set_crtc_gamma". When I change color profile for the first monitor, I get:
meta_monitor_manager_handle_set_crtc_gamma: serial=2 crtc_id=0
When I change color profile for the second one:
meta_monitor_manager_handle_set_crtc_gamma: serial=2 crtc_id=0
So I think that it's not Mutter's bug. Monitor #2 (closed) is configured as the primary.