Background color settings can be found in the "Style" section of the drag-and-drop editor. At the top of that section you will find options for the "Standard Background" and "Extended Background", seen below.
Standard Background: Standard Background controls the width of the email content itself. Whether you have that set to 600 pixels or a different width, this will be the default background of your layout cells (these can be changed on a cell level basis).
Extended Background: The Extended Background controls the background outside of the email content. For example, if a subscriber views your 600px wide email on a window that is 800px, they will have 100 px of open space on each side of their screen. This is where the extended background color will show.