Case hardening is not (IMHO) a DIY process. It involves a lot of heat, enough to possibly affect the thinner parts of a Mauser receiver. I say case hardening because that is the process; the color is a byproduct, a result of some case hardening methods. I suggest Googling the terms "case hardening" and "color case hardening" to see what is involved.
A simulation of case coloring can be done with a torch, even a propane torch, if care is taken. Good preparation is still required and the colors are not very durable, but for a display piece the result is usually adequate. I might note that real case coloring is not very durable, either. Case coloring is always (AFAIK) covered with a clear varnish or other coat to preserve it. That is why so few old guns that were case colored and have seen much use look good today. The guns with the beautiful colors that we see in the museums or in high price auctions are those that were put away soon after they were made and never used.
FWIW, case hardening was not originally for decoration. Up to around the end of the 19th century, steel could be made only in small quantities and at considerable expense. It was used for some gun parts, but frames were not made of steel, but of wrought iron or even cast iron. Iron is fairly soft and cannot be hardened or tempered like steel can. Further, it wears easily, both from outside sources (rubbing on holsters, for example) and from the motion of the parts inside, which were usually harder than the gun frame.
So, guns were case hardened (or carburized) to give a hard surface to the frame and/or other parts. Some of the processes used produced attractive colors on the frame. After steel came into common use around 1900, some companies (e.g. Winchester) stopped the expensive and time consuming use of case hardening, while others (e.g. Colt) continued to use it for decorative and indentification purposes, like a trademark.