4140 can be Colorcased with Bone just like Mild Steel, but the temps must be lower so that the metal does not reach critical.
Accept for 4140....Colorcased, bone Hardening, carburizing, etc. when done for color on mild steel---are ALL the SAME!
Generaly among gunsmiths Hardening means Hard with NO color!
Hardness on mild steel is usualy a .001-.004 skin with a soft center.
Factory Rugers are colored with cyanide and Turnbull's are done with bone. Take a look at them side by side, you can't fake real bone case colors.
I don't think Doug has made any miss statements in his advertizing. I'm sorry if anyone takes this the wrong way, but you guys don't really know enough about casehardening and how the process is applied to different metals, and a lack of understanding of the redundent terminology of gunsmithing to be having this discussion.
The OP's question was who to send his project too. My recommendation would be the same as Harley and John....Hunter Restorations
Raven