The color you see on your favorite rifle, shotgun, or handgun is part of makes it a thing of beauty often like a work of art. No matter how old or what method was used, the results can be astounding. Many a family heirloom can sit for decades in a closest only to be dug out and someone is disappointed because of cosmetic flaws sometimes so bad that there may even be pits in the steel. This often shows up first where the blue has worn off or has been scratched much like rust on you car.
Some of the methods of include “Hot Caustic Salt”, ” Slow Rust”, “Express Rust” and Nitre bluing. There is also Parkerizing which gives the best protection of traditional metal finishes and is often recommended under many spray on finishes. Bluing is primarily cosmetic with some forms of bluing (or parkerizing) providing a level of protection from the elements. All methods involve some for of heat/chemical change to the surface of the steel.