If the outside walls are drawn the same direction (reverse walls that go the other way), you can multiple select and change them all at once. Pretty fast for me.

But a function that would be cool, is if it were possible to assign a general name to the wall colouring (e.g. "outside panel") - and then change all walls in one go by just changing the colour/texture assigned to the name. And similar with floors, ceilings, furniture elements).

ok