Thanks for looking & commenting and showing your interest, Zeke Pickleman - @zekepickleman. I don't know, exactly, what caused the beautiful toning. Probably, some kind of oxidation, and technically damage, to the coin. Natural toning can take decades, and can be most desirable. Old coin albums can cause toning, like this:

! Natural Rainbow toning is where it's at, as opposed to artificial, "doctored"

It took me decades to appreciate, Mother Nature's, beautiful, colorful work. And I am always looking for rainbow toned coins, especially green & red, lately.