Not sure of it being an error, but toned yes. Looks natural to me. Going by the photos I'd say this one falls somewhere in an MS63 range. The reason I bring that up is, if you are not aware, in 1982 and 1983 the mint did not offer mint sets on a national basis. Meaning, they could only be bought directly at each mint in person. Philadelphia or Denver. They are commonly referred to as souvenir sets. And they are pricey.
Not an error--environmental toning. Possibly could have been lightly cleaned early in its life, with some toning beginning to take place afterward. I say that because--given the photos--areas immediately adjacent to lettering appear darker--perhaps some accumulated crud vs. the cleaner-looking fields.