On the 560 I know that the older ones are black and the later ones plain aluminum. Mine are black ('89), and at a junkyard while searching for other parts I found a 1991 560 SEL with aluminum covers. I suspect the black ones were simply painted.
Torquing is EXTREMELY important on these, by the way. I bought a 1/4" torque wrench just for these covers.
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)