As additional information and since you also collect coins, we can add that the toning, we can also see it, naturally, on the coins when a discoloration is formed or light patina due to oxygen and chemicals in the air acting on the metal. This oxidation can result in a variety and level of toning and also depends on the properties of the metal – silver, gold, copper, nickel, etc.
Toning in coins is a slow and normal process that can take months to years to appear. Should you worry about toning? If you’re storing your coins properly, don’t. Unless a coin is in a vacuum, it’s going to start at some point.
The toning and its color will depend on a coin’s metal composition. Older coins can show more color variations due to multiple and less pure metal "blends". And metals like copper and silver can react more quickly and often times tone easier when exposed to the environment.
Here are some very general toning characteristics of the major metals in coins we know today:
Copper – The life cycle you’re most typical to see is orange to reddish-brown to full brown to nearly black.
Nickel – Silver-ish to eventually a musty gray.
Silver – Bright silver to brown to black. Sometimes rainbow-style colors appear in the toning of silver, which can really add to value.
Gold – Bright yellow to orange. Sometimes a reddish color.

http://www.coinnews.net/2007/09/03/what-is-coin-toning-how-does-it-affect-coins-and-their-value-3565/
Greetings @neoxian