A toned coin is most often seen on a coin that was put away for a number of years in an environment that assists toning.
Mint and proof cent cello coins often alter the coin when they are stored for a long time in the cellos.
Sometimes the cellos were slightly opened and the colors can change/accelerate the process:
Found this one in a lot of 50 cello proof coins I purchased.
Matte Proof coins are often toned different colors:
Coin stored in a Whitman folder can alter the color of the reverse sides of the coins.
Even the better albums can turn BU coins in them. (This one was BU when I put it in the album, years later it turned:
Soft flips can turn coins purple from the PVC that the flips are made of. (Turn a purple color) Coins stored in a white envelope can turn them very dark in color.
Even the areas there there is a higher sulfur content in the air can turn silver coins yellow. (East coast area)
I prefer to keep my coins in 2X2's. But others like Hard flips that do not contain PVC. Even soft square can turn BU coins a different color. Just something you learn when your in the field for a long time. Some like toning, others like the natural color. Some even try to fake toning.
Coins can also be altered in the storage in the rolls they are stored in:
Even some forms of cleaning coins can alter colors of the coins in a negative way. Heat is one way to ruin a coin:
Some can be found in change, but who knows what happened to cause this:
So check stores in your area for toned coins if you like these.