Go to the paint menu in career mode in the workshop. Highlight "Metallic" and press X. Once in the color selection screen, press Triangle. Go to the base color and chose one. The original base color will now be active as a metallic color. For example, if the original base coat is red, if you highlight black as the base color the car will appear as black with a red metallic coat. This does not work if the car has been repainted already.