if they are chromed you can just paint the recessed areas satin or gloss black. If they are painted you can use the silver of your choice with black in the recesses. Eastwood company used to sell precut masking sets for Magnum wheels.
Since the Magnum 500 was not offered as original equipment on a 1969 Cougar, there is no specific paint scheme that would be correct. As yeloxr7 noted, paint them the silver of your choice and the inserts satin black which is what would have been available over the counter back then.