It would be nice to have different colours for the inside and outside of a building, or for adjoining rooms. Having a single colour of paint per block makes colour choices for a room interfere with whatever is on the other side of each wall. Initially, one colour per face seems sensible, but from a technical perspective it may make more sense to colour the edges/corners. Doors and windows provide cover for seams created by openings between spaces, and require the painted colour to reach around edges, leaving a painted seam on a face. The paint sprayer then would colour the four edges of the targeted face.