It may be toned silver, but it's still silver. Silver tends to pick up sulphur from the air and turn yellow or brown with age. Take a look at the silver dollar below that I'm posting on eBay today. You can find just about any color you want as you progress from the "silver" at the center to the blues and blacks at the rim. This was the end-coin of a roll that sat around for 50-60 years. Nobody worried about "sulphur-free paper" in those days [g].