It seems that I read somewhere that the way to color 4140 is a chemical process that imparts color to the steel without any surface hardening due to the properties of 4140 not being friendly to the heat involved involved in color case hardening. I don't recall where I came across that info or what the chemical process was but it seem like it required about 300 degrees F. so it would not draw heat treated 4140 to a softer state.