Go to the "faces" mode, select all the faces, open the material editor, and either make a new texture for them or use an existing texture; either way add this to the end of the material name -
That will make those faces the same color as the primary car color (if it spawns green they will be green, etc). You can also do this with whole objects. To make the parts the trim or secondary color (Sabre stripes, etc) use this instead -