I also have always assumed that it was due to the light spectrum. White light (visible light) enters black cars and is not reflected back at us except for the light's reflection on the clear coat, whereas lighter cars reflect back much more of the light, causing a loss of depth. Notice that when you apply certain waxes (NXT in particulat), the paint actually appears darker, which causes the perception for more depth and gloss. This is just going off of my chem/physics knowledge; this information is not absolutely factual.