White is never white, it's always gray, even in the 3d view, prior to rendering, the software assumes there is shade. If, for example, you think of a shading bar, 0 = black 100 = white, sweethome/sunflow only seems to use 0-70, that's how it seems to me, unless you use additional lighting to force white to show. Wait till you get to ceiling's, you'll love them.