Emmanuel:

I searched a bit, and found these two posts / threads.

http://www.sweethome3d.com/support/forum/viewthread_thread,1655
http://www.sweethome3d.com/support/forum/viewthread_thread,1441

The first post is an example you made yourself, showing two ways of making a multi-coloured room. Maybe it's a bug caused by some form of "code regression" ? When I load that file, it doesn't display correctly anymore.

Hans