The resulting case hardened part may show distinct surface discoloration. The steel darkens significantly, and shows a mottled pattern of black, blue and purple, caused by the various compounds formed from impurities in the bone and charcoal. This oxide surface works similarly to bluing, providing a degree of corrosion resistance, as well as an attractive finish. Case coloring refers to this pattern and is commonly encountered as a decorative finish on replica historic firearms.
As has been noted, many "color case hardened" finishes applied to firearms today are fake finishes designed to look
like traditional case hardened steel, but are not the result of real case hardening. Many modern firearms are often made from homogenous carbon steel and thus the entire component can be heat treated rather than just a hardened outer surface. This can negate the need for case hardening just the outer layer of the object. In this case, an artificial case hardening finish is applied for a historical appearance.