i believe that is to help you focus in the building you are working with (something to remove unnecesary distractions without removing the surrounding stuff)
also as penguin said, if the building you’re trying to edit is blackened or darker than the final colors then you might be creating a new one over the one you want to modify
I agree with this, I wish the building would show exactly how it is built. It’s really hard to color match with adjacent buildings and to match with the real world in this builder.