Acids can etch away the nickel from cupronickel, leaving more of the copper behind. That nickel-depleted metal tones darker than normal cupronickel. Pretty sure that's what's happened on this coin.
Since the composition is 75% copper and 25% nickel, various concentration of chemicals can produce patches of different colored products. If you ever had (a) drink(s) slosh over change in a cup holder, you can find various color combos after a few weeks Jim
Another possibility is a nickel in the ground (found metal detecting) can bring the copper to the surface of the coin. Usually, ground finds are not shiny but I've heard of some people putting their found change in a rock tumbler to clean it up to use.
@merry christine ellis Your nickel was slightly affected by environmental exposure of some kind Here are some of my metal detected nickels which the Cupro-Nickel was severely affected by exposure to the elements.
Naw, pennies don't melt with these, even when it's really, really hot. Circulation will eat away at these. They get to looking a little funny for it, sometimes.
Your nickel appears to have been laying, or covered, by something that brought out the copper color. Note the distinct pattern, like squares. In any case, NAV and not an error of any type.
