Right now its linear +1 to darkness per level but it could be easy change, right now I thin about adding vector of values from with engine will read bonus shade of levels under current one.
Something like that:
shadeLevel: [2, 2, 3, 3, 4, 4, 5, 5, 16]
Now with something like that first and second level under will be darken by 2. two next levels by 3, etc until 9th and any lower level where it will be pith black.
shadeLevel: [2]
all levels under will be darken by 2.
something like that will be useful?