Nickels are made of a copper-nickel alloy that is silvery in color. Unlike CuNi-clad coins (modern dimes/quarters/halves), nickels are the same color all the way through.
The color on yours is caused by something else and is most likely due to environmental effects after the coin left the Mint.
In Memory of Crazyb0 12-26-1951 to 7-27-2020
In Memory of Tootallious 3-31-1964 to 4-15-2020
In Memory of T-BOP 10-12-1949 to 1-19-2024