The Sharpie is indeed used for removing the bright shiny finish of the wheel rims, and the coupling rods, too. I have found in the past that airbrushed paint on these bare metal surfaces can rub off too easily, and even flake off if applied too thickly. I read, many years ago somewhere, that this could be prevented by blackening the surfaces before applying the paint. At the time I didn't fully understand the term, 'blackening', and used a marker pen. That looked blue to me, rather than black (yes, I did use a black marker!) so immediately rejected the idea. If I now work on something with bright surfaces that will be airbrushed, I will apply a coat of black (still looks blue to me) from a marker pen before airbrushing starts.
You might be able to see the difference in this photograph, where the wheel rims and coupling rod have been done, but the flanges have not.
Perversely, they look pink in the photograph rather than blue.