Predefined colors were mandatory when Sweet Home 3D wasn't able to import .mtl files (before version 1.3).
As from version 4.0, it's also possible to modify the materials of a piece of furniture from Sweet Home 3D user interface, I guess your idea to name materials from their function rather from their color is a good idea. I did it myself for a few 3D models last summer, and in the bunk beds model (and a few other ones), I even used different names for the materials of the two beds even if their colors are the same. This way the user have more choice of customization.
I wouldn't worry for the performance loss, it's probably not significant.