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The color of case hardening (but not the hardness, unless it is already there) can be easily mimicked with cold blue, simply by daubing it on. But another way is to use a propane torch and spot touch the metal with heat, just enough to leave a color but not enough to affect any hardening that is in the part. The part must be highly polished for the coloring to work. Different colors, ranging from blue to yellow to red can produced by using different temperatures. The color is not very durable, but then neither is true case coloring; manufacturers use a shellac or transparent varnish on the part to extend the life of the color.
Note: Because of the possibility of using too much heat, the above coloring should be used only when strength of the part is not a major factor. It is OK, to color the receiver of an old .22 single shot, but maybe not OK to heat color the receiver of a .300 Win Mag.
Jim
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