The straw color is the result of surface oxidation. The colors come and go through a temperature range, light straw is one of the lower temperature ranges, followed by a bright blue, a purple a dark blue and a black. The colors are formed by various mixes of ferrous and ferric oxides that are fairly consistent with temperature and the oxygen availablity. A heavier coat can be produced by longer temperature exposure.
I do not have my books with me or I could give you the temperature ranges. Color case hardening takes place when the part is buried in a chemical powder containg carbon, like bone meal or powdered charcoal, and some additional agents to make the rate of oxidation different on diffent areas. Carbon is driven ointo the surfface making it harder and the uneven distribution of coloring agents, like ferric cyanide, give the mottled color.
The color shift with temperature indicates how hot a piece has been and was used in the tempering process to judge when enough hardness had been "drawn" back out of a hardened piece to achieve the desired hardness and flexibility. Blue or purple are the right color for spring steel, note the color of the flat springs in the Luger mechanism.
It is possible that the straw color in the various luger parts had to do with a low temperature tempering or it may have been only decorative. The blur/purple spring colors are part of the tempering process.
The oxidative states which contribute to the colors can be unstable, especially for those oxides formed at low temperatures. Energy from UV, or infrared or even high environmental temperature could cause the states to relax back to the light gray. The higher heat oxides are more stable.
To do the straw process, you can raise the part to the correct temperaature in an oven or in a gas burner flame. About 400 degrees is a good place to start in an oven. If using the oven a coat of motor oil or lampblack on the part will help limit oxygen access which generally makes the color more intense. Put in a few test pieces of scrap to learn the oven temperature and time need for the color you want. Lower temperature and longer time is best. Using a live flame like a gas burner creates and oxygen poor environment around the part so a coating is not needed. Without the coating you can watch the color form. Immeadiately on removing the part from heat you must quench it in oil to cool the part and stop the chemical process. Otherwise it will revert to grey or progress to a blue. Be very careful. You may seriously change the hardness of the part.
Blues tend to age blacker, straws tend to fade. My observations of color case hardening is it tends to fade.