I think I understand what you mean.
Actually, all rooms are half transparent, but when you draw a room (like the 4.27 m² one, the 1.77 m² one and the other small ones) superimposed on another larger one (like the large green one), the small rooms get less transparent.

Sorry, I haven't understood your explanation, what the logic is here, why one room transparent, another not? How to get superimposed rooms always nontransparent?

I see there is a trick to get nontransparent room, superimposed on lager one - at least one corner of the room should be drown outside wall contour - now room became nontransparent, then you can correct it as you wish and it retain colour. It works, but then I cannot use doubleclick for fast creation room inside walls, it was very comfortable, rapid drawing is important for me