Is this a copper pigtail?

Enough about toilets for me because that sucker is down and out, lol. Since the bathroom being redone is mainly used by the kids I was going to pigtail in copper to the outlet and install a gfci receptacle. When I pulled out the old receptacle it was hooked up to a pigtail already and the color of the solid wire was copper. Nothing I have read shows aluminum being any color besides silver. I used the exisint connections and the outlet works fine. Any input appreciated as usual.
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Some aluminum is copper clad. It solves the oxidation problem, but not the expansion/contraction problem. It should say on the wire what it is.
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