I don't know too much when it comes to this.
But I'd say the game see them as textureless, hence the black and yellow.
You should make sure you actually enter the correct material name for the meshes, and if you're really sure they are correct, it wouldn't hurt checking the .mat file for the meshes and possibly the textures it wants to use. If the names in the materialfile is written wrong or the actually texture picture is missing, this probably happens.
I could be wrong though.