
Kathryn S. answered 07/08/19
Musician and Former Music Major
A single chord that's out of the key doesn't change the piece's key. You can use a chromatic mediant as a pivot chord to move to a new key, but a. that's not a chromatic mediant anymore, it's a pivot chord, and b. you'd have to continue on in that new key using a cadence or common chord progression etc. to help the listener understand that the key has changed.