Setting the background color for a box or group of boxes is a great way to organize your plans and make it easy to find things quickly.  You can set the background color by clicking the Edit link in any of the boxes.  Look for the icon with multiple color swatches.

Once you select it you can choose from 16 background colors.  Hovering over the color will give you a preview.  Click the color to select it. Clicking Reset will restore the box to the default white background.

Once you've selected the color you'll see it in the edit window and as soon as you save it will be applied to the box.