sflem is correct, they were galvanized when new. for a low cost color, you can use rustoleum silver aluminum paint with some valspar hardener, looks pretty good. i used a gm ralleye wheel paint for a cadillac escalade and shot in base clear. here is the m with base clear on the wheels, red is crossfire.
here is a farmall cub rim with rustoleum and hardener. the inside is painted with por-15 conversion primer. i loaded the cub tires with chloride for ballast