sudo xrandr --output eDP1 --brightness 0.5
I use values down to 0.2 on my chromebook. Colours wash out at low values but you might be able to adjust them separately (I did this once but can't remember the details).
Choice is much more likely to be implemented if at least one of the people wanting the choice is also able to implement it.
My assumption is that 'UBports Orange' is set in only a few places. It is no disrespect to the developers to ask their advice.