[GrabacrOne]

The hat is shiny because the OE BMW rotor is a 2-piece riveted design where the hat is aluminum (which doesn't rust but can corrode over time) and the friction disk is iron. Some aftermarket and OEM suppliers also have a 2-piece product, including Zimmerman, Brembo, and Pagid. The 1-piece design has the rust-inhibiting zinc coating on the rotor hat as well since it's iron, which gives it that dull gray look that will eventually come off. You can powder coat or paint this part to keep the iron hat from rusting.

I would check the product catalog for the part number of the 2-piece rotor and try and get that. These are typically more expensive - often twice as expensive - as the 1-piece one. There are arguments over which is better, but I try to stick with the OE design which is 2-piece riveted for the stock floating calipers.