Generally, the oxide layer is black, but it can also have other annealing colors (such as yellow, red and green). The color depends on the temperature of the heated layers.
The material surface remains even during annealing because nothing is removed from it; the color change is achieved through localized material heating only.
The heat usually penetrates 20 to 30 µm deep in the metal surface. This makes the marking non-abrasive and ensures that it can be removed by re-heating (min 700°C for ferrous metals).