Color case hardening. It is done with bone charcoal usually, Carbon rich, oxygen starved heat.
Here is the frame to my 44 Stevens that I just got colored again. This is the real deal. It is posted in other places, but just so you can see. I am pretty sure that NEF does color case harden their frames too. I have several and they have genuine color case hardening.
Case hardening, or surface hardening, is done by placing the parts to be colored in a crucible with charcoal and high carbon bearing substances such as ground bone, leather, animal hooves, then the crucible is sealed shut, and brought up to a critical temperature (somewhere around 1600*) where the compounds in the crucible react with the surface of the steel to provide the colors. In the case of the 44 Stevens receiver, the receiver is stood on end and the compounds are layered in to obtain the alternating layers of colors. There is lots more to it than that, but this is the simple picture. The colors appear as a reaction of the heat and carbon in a carbon monoxide atmosphere in the crucible.