Good news - bad news. The good news is that the "tarnish" will not affect the cooking qualities of your Magnalite cookware one bit. This "discoloration" is caused from a reaction with chemicals that were in the water - you can get the same thing from the chemicals in the water when you boil water, or when you boil eggs.
Bad news - you'll probably have to live with it unless you can find a place to commercially refinish it (the commercial guys have access to chemicals and equipment the average person doesn't) ... and that could cost more than you paid for it. It might be as simple as a dunk in an acid bath ... and then repolishing ... I don't know. You'll just have to break out the phone book and call around to metal refinishing shops and talk to them.
Two ideas to try ... wipe it down with isopropol alcohol ... a couple of times and see if that removes any of the tarnish - repeat if it works but I doubt it will remove all of the tarnish. Then, try washing it well with some Bar Keepers Friend (follow the instructions on the can). I doubt that you will be able to return it to the "pristine" condition you remember without some commercial help.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain