Brake disc paint is for people who regularly track their vehicles (typically used in racing) or have vehicles that are hard on brakes in which it's a good idea to keep brake disc temperatures in check.
Aside from a pyrometer, thermometer, or infrared sensors, Brembo Racing rotor paints are the most cost-effective solution to managing brake disc temperatures.
Once they reach the specified temp range, they oxidize which turns them green/red: white or an orange/off-white (yellowish tint). If they reach the peak temperature.
Tech tip: disc paint should be painted liberally onto the entire width of the disc including vane and rotor cheeks.