coins do tone those colours, but depends on what they react with. it is the way they tone though, I find that the top grade silver with a bit of handling tones green/yellow around the legends and it fades as you go in there is no way you should be getting a coin that looks like someone spilled oil on a road naturally, except when they are stored, there is a toning when they are storred in rolls on the top coins. I have seen rainbow toned brass, and that wasn't fake. if you look through silver from the early 20th century, you can see the toning always starts on the outside and works in, although there are examples in North america of a more uniform tone.