I took out my prewar PPK magazine, the finish is very uniform.
Change in color should be related with maker and time. The example that I posted above was a Mauser magazine, not a Walther. For E/135 P.38v marked magazines, there are more color change examples. For example, this one listed on pre98.com,, it is original in my eyes, here is the link, please note the color change on the magazine body in the last picture of the ad:
M250 MAUSER E/135 P.38 MAG | Pre98 Antiques
I visited numerous auction preview sessions over years, but found all auctioneers displayed their guns without magazine, obviously, they have safety concerns.. but that also make magazine observation impossible for the viewers in the sessions.