This sounds to me as if the texture files are not in the right place, because then you see trackside objects without textures and that's kind of a disorienting, monochrome mess of shapes...
If you're manually installing a track, you need to make sure any .jam files the track comes with are placed in a certain subdirectory of \gamejams\. The name has to be exactly as specified in the gp2form, otherwise the game doesn't find the textures and can't display them.
Another possibility could be that texture files were placed in the correct folder, but need to be transferred from .jad to .jam format with a convert.exe.