Not copper, not an error. This is what some circulated coins look like - it's possibly that there was liquid on the coin that dried, or something similar, but it's not copper from the Mint or the minting process. That color you see occurred after the coin was already well-circulated.
War nickels have 56% copper but they are an alloy, mixed with 35% silver and 9% manganese. So that can't be copper, it's already blended into the coin which is a more gray color than copper color.
That being said, would it be possible for the silver and/or manganese somehow be lost due to environmental considerations and for the copper to show? @Fred Weinberg