I think I better explain. As I know it , there are two types of production tires;(1) aluminum rims and (2)steel rims with Bundt hubcaps.

Bundt caps cover the entire inside surface of the steel wheel rim, so no brake dust can come through the gaps you have with an aluminum rim. They never appear dirty.

Aluminum wheels look much prettier but they have gaps in them through which brake dust gets through.When they get gunked up they look pretty ugly.