One form of laser marking is called annealing marking. In this case, only the surface of the material to be processed is edited by the laser. Metallic surfaces (precious metals) are specifically heated with the laser beam and thus obtain a discolouration that can be represented with different parameters as a colour spectrum. You can also observe this phenomenon in your car's exhaust system.
The achievable result depends on the carbon content of the material and the temperature of the laser. Because, depending on the material, the annealing marking will produce oxidations that differ in their colour.
Marking by annealing is ideal for all types of metals that change their colour under heat and the effect of oxygen. The outward layer of the material is only selectively heated by the laser beam and thus discoloured. That obtains an extremely high contrast and a very readable labelling.