At one point Krieghoff actions were reportedly Bohler chrome-molybdenum-vanadium but now are (likely - they ain't saying nor is Beretta's "Tri-alloy") AISI 4340 Cr/Mo/Ni
It is hard to obtain bone charcoal case colors with 4140 or 4340.
AISI 8620 (also Cr/Mo/Ni) is more easily case hardened (carburized), or nitrided

Reportedly Krieghoff was using some form of chemical coloring for the actions that are not silver nitride
https://www.trapshooters.com/threads/krieghoff-case-color-hardening.260887/

K-80 Super Scroll possibly chemically colored - no 'ghosting'

[Linked Image from photos.smugmug.com]

This is likely bone charcoal with the 'ghosting'
https://www.dupontkguns.com/kriegho...ardened-k-80-receiver-iron-only-135309r/

[Linked Image from photos.smugmug.com]

Lots of examples here
http://www.paxtonarms.com/Krieghoff.htm

https://sites.google.com/a/damascusknowledge.com/www/home