Here are mine.
Taken is 2005. I just had them bead blasted. This is what the centers looked like in 1992 when they were new. No coatings, just raw center section:
I coated them with acrylic clear right after that shot to make them easier to clean. Brake dust does not attack the clear-so just brake heat grays them.
Here in 2012. They turn medium gray from heat-moreso in front on an ERA.
They are only cleared all this time and cleaned with WD-40 towels-plus all the rim polishing. WD lifts the brake dust easily except it's a bb to get all the nooks and crannies-especially the backs.
The inner side gives a good idea of the natural aging. Much of the color change also depends on the mag alloy being used. Mine were advertised at the time as having a 'purer' mag content.
The natural graying is probably why ERA (or Trigo) powdercoats the ally ones gray when new.