Brass, copper and other copper alloys can be nickel plated to get a durable, shiny silver coating that is not subject to tarnishing.
Nickel-plated tokens have the same weight and electromagnetic signature as the non-plated tokens, so plated and non-plated can be mixed without reprogramming the the coin validator.
Virtually all alloys can be nickel plated, but nickel plating is mostly used for non-silver colored alloys, such as: