First, welcome to the neighborhood @Olaf lord ! Once you learn how coins are produced at the Mint and how to grade coins, you'll quickly realize that there is nothing unusual about this circulated coin. Chris
There's no error. The metals will change color after time. They can get dirty, they can oxidize, they can "tone". It's all normal. 25 cents. Since this isn't a valuable coin, take some toothpaste smear it on both sides of the coin, rub it in between your fingers, wash it off, dry it with a paper towel. I bet it becomes a lot cleaner.
