A brand new rifle, shot once, with copper or plastic fouling??? Just can't have happened unless you have a burr in that barrel the size of my Aunt Ida's bunion.

What you probably see is an accumulation of oil. Oil will change colors once in a while. Take a dry patch, put some alcohol on it. Swab the barrel clean. Then a couple dry patches. Now take a clean patch, add several drops of oil, and swab the bore in short strokes, really making sure you work that oil in the bore of the rifle. Then a couple quick up and down strokes to the excess patch and pull any oil it finds and you are fine.

You have new rifles worries. When I get a new rifle, it seems I am checking that rifle over and fretting over it for a couple day terrible. All that will pass.