See:
https://en.wikipedia.org/wiki/Bluing...)#Niter_bluing
"the colours will range through straw, gold, brown, purple, blue, teal, then black."
Obviously, this bluing process was not used for the traditionally strawed parts of a Luger, but it was used on the grip screws.
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.