I would if I could understand how you create your models.

If I were to assume that you create furniture using walls etc in SH3D before you copy-as-new / export-import - you must assign colours/textures to the original walls before exporting-importing. If you do, the different colours/textures will be saved with the model, and show up as wall-1-1 etc in the modify furniture dialogue. Other pieces of furniture will retain their original surface names (like the cylinder).

But I am making assumptions, and this may not be very helpful...

ok