Bitmap Data Help


I have a sprite with three children which are all shapes. Im using BlendMode.DIFFERENCE to make lots of pretty colours appear. (See attached swf). The problem I am having is that because the blendMode is set to difference it is causing the overlap of these three shapes to show on the screen as black. (Again see attached swf). What I would like is for all of these black pixels to become white pixels. If I copy the parent sprite into a BitmapData object will I then be able to do something such as:

for each(var pixel in BitmapDataObject){
if(pixel.color == 0x000000){
pixel.color = 0xFFFFFF;

Or am i going around this the wrong way, should I be using pixel blender?

Thanks in advance for any help on this