That color is standard for a coin that spent time in the ground. It's known as ED or environmental damage.
Welcome to Coin Talk! Thanks for sharing this coin! The posters above are right. It's environmental damage. Maybe you think you have an experimental planchet error? I'm not sure but a lot of advice you'll get on here is to read before you hunt for coins and errors. Buy the book. Learn how the minting process works and just plain use google and the great knoweledge here if you're not sure. We love to interact here and are more than happy to help guide you on your way but learn and understand the minting process and you'll be on your way to a great start in the hobby!
N A Nickel is made of Cupro-Nickel. The Copper tones when exposed to a harsh environment such as soil, dirt, sand, and water. Here are metal detected Nickels I have found.. Also affects clad coins made of Cupro-Nickel..