Actionscript 3 :: Change Pixels In An Image?

I actually try to do the following: I have loaded an external image in a bitmapdata object and create a bitmap from it which i attach it to a sprite/MovieClip in order to have mouse events on it. Now under the previous logic i loaded two images (let's say circles) of the same size one that has a particular color and is covered by its black foreground circle. When i press left mouse button and hold it down i want while the mouse is moved to erase the foreground circle's pixels and so the background image starting to appear.[code]...