New rotors come bare or with various protective coatings, and the coating affects how they look and resist rust - though never on the braking surface itself. Here are the common rotor coating types and what they actually do.
Why coat a rotor at all
The friction surface of a rotor must be bare cast iron and will always rust and be scrubbed clean by the pads - see why new rotors rust fast. But the non-swept areas - the hat, the edges, the vanes - can hold cosmetic rust that shows through the wheel; see coated vs uncoated rotors. Coatings protect those areas.
Common coating types
Bare (uncoated) rotors are cheapest and rust fastest on all non-swept surfaces. Zinc-plated or zinc-coated rotors resist rust well and are common on quality replacements. Geomet and similar proprietary coatings are premium finishes offering strong, long-lasting corrosion protection with a gray look. Some cheap rotors just have a paint that burns/wears off.
Where the coating survives
On any coated rotor, the coating on the friction surface wears off in the first few stops as the pads sweep it - that is normal and expected. The coating that lasts is on the hat and edges, keeping them from turning orange. So a coated rotor still shows bare, shiny metal where the pad rides.
Does it matter?
For braking, no - coatings are about appearance and rust resistance on the non-braking areas, plus a bit of storage protection. For a car with open wheels in a salty climate, a zinc or geomet coating keeps things looking clean; see how to prevent rotor rust. Pair a quality coated rotor with good pads and hardware.