The background color of each article can be set through the article properties screen. (menu⁄format⁄article). For this, check 'This article has its own background color' and select a color in the color combo directly below.
It is also possible to assign the background color to all articles in the selected subtree, for this check 'Apply to entire subtree'.
If the article has no background color ('This article has its own background color' is unchecked) it will show the background color as specified in the options screen.