To Reposition a Color Key on a Gradient Fill

From the Active Color dialog, select the Gradient Fill tab.
From the Type option menu, select the fill type.
From the Color Mode option menu, select Multiple Colors.
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: To move multiple keys simultaneously, hold the <Ctrl> key as you select the keys. When you move the keys, the space between them is preserved.

Note: If you drag another key all the way to either end of the gradient color bar, then that key will replace the end key.