Maybe letting the player select a ceiling swatch for the whole house, per level, or per room could work. It would probably function similarly to the roof tile. We can have white, grey, black, wood tone swatches to start. (This sounds simple to implement, but it's probably not.)
Or whatever floor tile was used on the higher level could appear as the ceiling of the lower level. (Definitely not as simple.)
I also thought of having the ceiling match whatever wallpaper you put in the room. But it'll be harder to do that because you can have different wallpapers on different walls within the same room. So I'll stick with my first 2 suggestions. :))