This inverts the transparency on a Bitmap (probably supplied a PNG source) that specifically has two colors in it; "#656565" and "transparent" - you can tweak the threshold lines specifically if you'd like to modify that to use a range, or whatever.

Adapted from http://stackoverflow.com/questions/608087/flex-actionscript-white-to-transparent and http://www.sephiroth.it/tutorials/flashPHP/thresold/