Unless you have directly defined the backgroundColor on the element itself, you have to use getComputedStyle()
or currentStyle
to get the value of a style property.
A method that is compatible with multiple browsers would look like this:
function getStyle(el,styleProp)
{
if (el.currentStyle)
return el.currentStyle[styleProp];
return document.defaultView.getComputedStyle(el,null)[styleProp];
}
You can see a working example on jsFiddle.
More information: