I had a nickel that was pinkish. It looked quite nice. Figured it was marker play. Gave to grandbaby.
mint error - minted on a 2018 rose gold breast cancer awareness $5 gold planchet https://catalog.usmint.gov/breast-cancer-awareness-2018-proof-5-gold-coin-18CE.html or maybe urine stains (hey, don't point that finger at me, that's in another thread).
The metals in the coin react to air, water, soil, chemicals, pollution etc. and can be a variety of colors when exposed to the elements.