Color coded rim or plain black, why ?

694spd said
Here is my spin on the subject. If you ordered a full wheel cover then the wheel was painter black. If the car came with the standard "dog dish" hub cap then the wheel was painted to match the body color. This all changed sometime during the 1971 model year. The factory made a production change to now paint all wheels black regardless to the wheel trim.
I sure others will reply with any needed corrections.

This is correct. The line change was somewhere around May, and I would assume as a cost cutting option. My car was built 07/09 and had black painted steel wheels.