Looking at the original image, the discolourisation doesn't look uniform as "White bits" are showing through, (The original colour?), this suggests to me that the medal may well have been "Oversprayed" with a protective coat of - say laquer to keep it looking new. this laquer may have discolourised due to environmental reasons ( Smoking etc etc) and broken up revealing the clear surface underneath.
Solution, I would suggest as others have, a mild cleansing in a soapy solution, although I doubt this will work as the original surface finish is permanently contaminated. You then have to choose to either remove the original laquer (Drastic" to reveal the true under finish or leave alone and accept as mentioned before that in this case it's merelly showing a part of it's true history. depends on which is more important to you the medal, or a prestiegne example of the medal. If it's the latter wait until a "Mint version" comes along and then buy it.
regards
Alex K