Interesting. I believe the two tone magazines are all pre-war also. But you probably knew that. I have an ACE magazine lying around that is two-tone. No gun, just the magazine. /forums/images/graemlins/smile.gif
I had always been under the impression that it was falsely believed that the blueing process weakened the magazine feeding lips, so they dipped them into the blueing mix base first, holding the mag by the top and not putting them in all the way. I have never heard of the cyanide dip theory. That's what a gun dealer told me, anyway. Either way, I believe all Colt mags were two tone up to 1940. They continued to issue them to GI's until they ran out, around 1943. I'm not sure when they switched commercial mags to one tone.
The reason from what I remember was in the .22's it was polished for "feed/fitting" and in the centerfires it was due to the 'heat treat' and to maintain the "shape" of the side walls over time. So dipped, stripped, polished , whatever, the early ones by Colt , as well as others i.e. High Standard, they were two tone.
The slide coloration is due to 'heat' treat at the end and where-ever there is part engagement, like the takedown notch area, you usually see these area a bit 'darker" due to the heat treat and the reaction of bluing or parkerizing.
So back to the mags, I'd have to say it's the same thing.