In TR3 some faces were textures with a color only (something that is just not possible in TR4-5) instead of a texture. When Core "ported" their model to TR4, those faces have been removed, so that's why they need to be added back. Or the mesh to be re-imported, I think our custom tools automatically convert colors to textures.
Also, this is not the right place to ask, this thread is for suggestions and ideas of tutorials.