Hi,
This is a problem that is often seen at physical Windows 10 setups as well.
On a Win10 VM that I run under Fusion I saw the same thing, so I searched a bit and bumped into this:
See also:
https://www.reddit.com/r/Windows10/comments/64cmp9/windows_10_cu_night_light_settings_greyed_out/
I then navigated in the registry to the mentioned key and removed the data from it.
That indeed enabled the night light settings.
However it did not appear to make any difference as the screen color and intensity stayed the same. I have not rebooted that VM yet, so that might be another reason.
Perhaps this is a setting that should be made at host level (something that does seem to make sense to me, although I would like to set it at VM level )
--
Wil