I'm not a Luger expert, but colors from light straw to deep purple are possible/indicative of the heat applied to steels. These temperatures are in the range where 'tempering' or 'drawing' (softening) takes place on fully hardened steel. So, the process was heat a steel of sufficient carbon content to above the critical temp (typically 1425F) and quench it. This is now a fully hardened steel. The hardness will depend on the carbon content as well as some other alloying agents. Then the steel is polished and then heated slowly to the tempering temperature. In the days before temperature monitored ovens, the surface color was the indicator.
http://www.anvilfire.com/index.php?bodyN...20anvilfire.com
