The OBJ files interpreter of Sweet Home 3D doesn't display the material colors you chose because Sketchup didn't create a new object with a line starting by g or o, for each color or texture change happening with a line starting by usemtl.
As there doesn't seem to be any OBJ export option in Sketchup that would say "Separate by color" or "Separate by material", I guess I'll have to update Sweet Home 3D...
At this time, I would:
- export the 3D model in DAE/Collada file format from Sketchup,
- import the DAE file in an empty home in Sweet Home 3D,
- export it with 3D view > Export to OBJ format menu item,
- change in the generated OBJ file the name in each g line preceding the usemtl matching a mirror material, with something prefixed by sweethome3d_window_mirror,
- import the modified OBJ file in Sweet Home 3D again.
Hope this is clear enough...