It is patina, or oxidation in scientific terms, if you keep it sealed and away from air it will slow, or stop the oxidation. Humidity, temperature, etc all have a role in the way they patina, as well as other chemicals etc that carry in the air. Sulphur contained in the paper a lot of coins were stored in is well known for creating beautiful toning over years of storage. To some toning of any kind is undesirable, to others, they collect the more beautiful toned specimens. Personally I like colorful toning, but would still prefer the coin in the condition it left the mint in.