It has a copper inner clad layer like all post-1964 dimes, but the outer layers are copper-nickel. And the reason it's that color is due to environmental damage. It is merely discolored, in other words. That one spent some time on or in the ground. Metal detector finds will often come up looking like this.
The slight toning is from environmental exposure to the elements. Soil, dirt, sand and brackish water will do that. Nickels are also composed of Cupronickel Planchets so they also tone just as @lordmarcovan explained in his post. Here are many examples from my metal detecting finds.. And Kennedy Half Dollar
