There are a number of places that the color could be set, in decreasing order of importance. Colors established in your theme's typography.css (which is used in editing mode) could be overridden in these:

You're looking for the background or background-color style.

Inline CSS on the element.
Block override (look in block options)
Block style sheet
Main

To find out what style is actually being used, in Firefox, use Firebug to examine the markup. The right column will identify the various stylesheets and styles in use; the background/background-color style that is not crossed out is the one being used.