Aluminium oxides nearly instant upon being exposed to the air. Unlike iron that oxidised layer protects the metal underneath from further oxidisation. Anodising makes that protective layer thicker, can add color etc. Also the cases can be colored by using a dye and sealer.
Apple typically uses a process that uses anodization, dye, and sealer (in that order) Mac Pro might not have dye on it, something like a fluo green iPhone surely has dye on it.