The master cylinders for the 100 were cast iron and painted black but the 6 cylinder cars with separate brake and clutch master cylinders are unfinished cast aluminum. Since the BJ8 master cylinders currently supplied are cast iron what ever you paint it will not be correct. Aluminum paint color will look more like aluminum but if you really want to be correct in finish then you should find a used aluminum master cylinder and have it resleeved. Hope that helps a bit!
Cheers,
John