[background transparency only]

Note: The following does not work in IE8

 

I came across this just the other day, but the solution I was looking for would ideally not have me define the background colour in both the CSS and JavaScript. In the end I managed to solve it by using the "getComputedStyle" function, and then modifying the alpha value as required.

 

In my scenario I was trying to fade in the background:

#el {
    background-color: #000;
}

var $el = document.getElementById('#el'),
    bgColor = getComputedStyle($el)['background-color'],
    fromBgColor = bgColor.substr(0, bgColor.length - 1) + ', 0)';

TweenLite.from($el, 0.3, { backgroundColor: fromBgColor });

For this to work I'm working on the assumption that "getComputedStyle" will return me an rgb string for the background colour. I'm currently trying to figure out whether this is a safe assumption to make.