I don't know what the discoloration is, but it looks to me as if it's merely an oil sheen.
Did you clean off its new-gun factory coatings, as you were supposed to? Maybe shooting it got the factory's oil or grease warm enough to make it show.
Using a gun gets it dirty, and makes wear markings on it.
Some people prefer that, because it makes the gun look as if it's been used.
Find something else to worry about.