The centers are going to be HPC coated silver the same as the side pipes.
I restored Garrett's (Justbetter)wheels a little while back. I stripped them down and had the rims re polished and sent the centers away to be HPC coated. They came up magic. Probably as close as you'll get one of these cast centers to looking like polished aluminum.
I'll see if I took a pic of them. Otherwise I'll call in and see Garrett and snap a pic. I think he's using them as a fancy coffee table
Cheers