Pull a wheel and find the rotor tinged blue or purple, or spotted with dark patches, and you are looking at the fingerprint of serious heat. Rotor bluing tells a story about how the brakes have been used. Here is what it means and whether the rotor is done.
Bluing means high heat
Cast iron changes color when heated - a blue or purple tint on the rotor face means it reached very high temperatures, often from hard or prolonged braking, a dragging caliper, or riding the brakes down a long grade; see how hot brakes get. The color itself is just oxidation from heat - but it signals the rotor saw temperatures that can also cause other damage.
Hard spots and hot spots
Severe overheating can create hard spots (martensite) - localized areas where the iron's structure changed and hardened. These raised hard spots wear differently than the surrounding metal, causing thickness variation and a pulsation you feel when braking; see what is brake judder. Hot spots are a common cause of a shudder that resurfacing cannot permanently fix.
Is the rotor ruined?
Light bluing with no pulsation and plenty of thickness left may be cosmetic and fine to keep using once cleaned up by driving. But bluing with hard spots, cracks, or a pulsation usually means the rotor is compromised and should be replaced - see when to replace rotors. Machining rarely cures true hard spots.
Fix the cause too
If a rotor blued from a dragging brake, fix that sticking caliper or the next rotor will do the same. If from hard use, consider better pads and fluid. Replace damaged rotors as a pair with quality pads and fresh hardware.