Click on the vertical bar for the desired color key, and drag it to its new location.
You cannot drag a key past another key, but they can be abutted to cause a sharp edge between the adjacent colors.
Note: The first and last keys of the gradient cannot be moved, but if you try to drag them a copy is made of the key. If you drag another key all the way to either end of the gradient color bar, then that key will replace the end key.
