@herf Hi! I came here to add my +1 to this feature… Even though it wouldn't figure out sunset and sunrise times, I was quite happy with having added two cron jobs that pointed to two shell scripts that pointed to two Shortcuts (quite convoluted, I know) that toggled colour filters on macOS, as a bit of a workaround for this shortcoming in f.lux, only to find out about this limitation.
I would much prefer for this to be an option in f.lux as well, as a lot of people who use it and Night Shift seem to be turning to greyscale mode as well. On the iPhone and on the iPad, they mostly use Night Shift, as sideloading f.lux hasn't been an option for years, but on the Mac, having the option to use f.lux, I reckon it would be quite popular as well. As a matter of fact, with alternative app stores and sideloading on the EU, maybe f.lux could come back to iOS and iPadOS?