The colour seems to have changed throughout the years. The wheels from the 60s/70s appear to be a light flat grey, while the later wheels from the 80s on wards appear to be a much darker flat or satin grey. Whether there have been just these two variations, or many variations of grey over the years, I can't say. For early period builds, I go with the lighter grey. Greg's suggestion to use primer for this would work well. Additionally, they are frequently painted and could conceivably be any shade of grey, or any other colour for that matter. There may be actual specific colours for the different eras, and perhaps someone will chime with specific knowledge. A flat or semi-gloss finish seems pretty much standard across the board.
On the two examples below, the Torino is wearing an unknown dark flat grey, and the Mustang is wearing a lighter custom mix of Tamiya Tank Grey and Flat White acrylic, approximately 70/30 mix.