It's copper, which goes from the color of shiny pennies to the color of dull, brown, old pennies with time. Expect it to happen, understand that the cooling isn't very different at all, and get on with life. Alcohol to remove oils is a good idea, but fretting about minor discoloration of copper isn't worth it.
Metal polish would be able to remove it... but you actually run a greater risk (IMHO) of leaving little bits of grit on the surface - which is worse.