No problem/Geen probleem.
The BoxLight is pretty much used as a 'minimum amount of light' light source, pretty much the amount of light left when most lights are out, though if you do not plan to make any more lights turn off in the scene you're making, it may be a tad brighter.
But, for example:
You have room in which only some light is shining through a window, and maybe only one or two candles are lit. The BoxLight in this kind of room should be subtle and prevent the scene from being so dark that it all seems pretty much black. (Though it should still be dark enough in the corners where the light does not reach that the player still has trouble seeing.)
Better said, the BoxLight is best to
only be set a bit or much brighter if there is brighter upto clear day light coming from non-covered (i.e. Curtains.) windows. So pretty much always should they be set really dark. As for the color, this depends on the scene itself, best to let the color of the scene in general decide the color of the BoxLight, perhaps even some of the PointLight that do not represent reflected light from other colored sources. (Like candles.)
So, green icky sewer? BoxLight leans towards green. This is of course decided after the majority of lights is added, as walls may be gray originally, but if the blue light that shines because of the night sky through the windows, the scene may look blue, so the BoxLight is best also set to lean towards blue.
Again, hope it helps, tend to keep ranting on, but I can't explain it with fewer words.