The finish is supposed to NOT be slick, from the factory. More of a duller silver. Some kind of a non-paint coating. I concur, they do NOT polish. When the wheels reappeared on the '77 Z/28s, they were painted body color.
For a close-match paint, I used GM Code 13 Medium Greystone Metallic ('78 or so Chevy pickup truck color) in DupliColor spray cans to paint the '78 Z/28 wheels I got, to change them from factory dark blue (take-offs of a '78 Z/28) to the darker silver metallic. Worked well and looked better than the earlier duller silver coating.