I have the stock 3 spoke rims on my sons convertible. We took them off to change the shocks and clean the rims of what appeared to be 13 years of accumulated brake dust.

On the back side one was aluminum and what appeared to be clearcoat.

Another had grey paint it appeared to be a medium grey but different from the paint in the openings with a lighter grey primer underneath. No clear. guessing someone attempted to paint it. Or did Ford do some rims that way?

 

My old 3 spoke rims are basically solid brake dust and dirt. Upon cleaning them off it just looks like a dirty aluminum.....I don't think Ford painted them the way you're saying. It does sound like someone painted it, but it depends on which 3 spoke rims you have.

 

Why would someone paint the back of a rim anyways?

EDIT: Funny..until this thread I had not seen any 3 spoke wheels in forever. This yesterday at Krystal's I see them on a Mustang. They actually looked ok. I swear there were 4 older Mustangs there at the same time. this town is Mustang poor lol.