Coin that alter the surface on plated coins, damages/alters the plating.
Even saliva or a coin sneezed on will be altered:

We've seem the darker colored red coins in circulation, probably a child at one time had it in their mouth.
Sneezing leaves fine darker spots on a BU coin. Finger prints also etch into a cent if not removed immediately. When start to see them, then it is too late as they are etched into the surface of a coin.

When I purchased this coin at a shop, there was no fingerprints on it. I just remember the person who found the coin said that the wire was still attached to the coin in the bank bag. He touched it and the wire fell off. So I put it away in a 2X2 and after a few years, there was the finger print. So I then believed the story.
So things added to coins will alter the color. Even dipping them in something acidic will turn a dark copper coin pink. (ruining the value of the coin a large percentage. Water left on a copper coin will add the green to the surface and also lower the value of a coin. So if you don't know what you are doing with a coin, don't alter it. Practice on common coins in change to see what is possible. Only when you get a technique, even try more coins to make sure it works. What you do to a coin today can ruin the value as a collectable in the future to your coin.