These dimes are not plated. It is "clad" which is a sort of sandwich. There is an inner core of copper. The outer layers are an alloy of copper and nickel. Your coin has toned. It's a sort of a rusting of the metal. For example, of it had been in the ground and was recovered (metal detecting) the color you are seeing is because of environmental damage. It's worth a dime, you can spend it.
