Is there a way to Night Light Filter or Grayscale without installing anything in dom0.
If absolutely no, then what is the way with the absolute least amount of running code possible after installing packages?
I think experts are needed here, and it absolutely deserves! Look at how many Qubes users are installing e.g. Redshift in dom0 in order to achieve such goals. I believe Redshift is not “least code” by any means.
I wrote about a method using picom. Unfortunately I never found a way directly through Xorg.
picom requires the library libev, you would have to disable xfce compositor (alt+F12 by default IIRC) and run picom instead.
you could run redshift without redshift-gtk, it is a much more smaller codebase.
An alternative to redshift is sct, a very small utility written by an OpenBSD developer: https://www.umaxx.net/ it is not dynamic like redshift, it just changes the color when you run sct, while redshift slowly changes the color depending on the time of the day.
Some desktop screens can be set in black & white if you set saturation to 0.
If you’ve already set your hardware backlight to the lowest brightness, you could use xrandr to further dim in software:
$ xrandr --output FOO --brightness 0.5
(Where FOO is the name of your output, shown by running xrandr without arguments.)
--brightness 1 resets to the normal, unfiltered level.
If you use an external monitor, you can adjust the settings of the monitor itself.