[silin]

можно поиграться с blendMode ластика и отрисовкой результата в битмап

package 
{
	import flash.display.*;
	import flash.events.*;
 
	public class Main extends Sprite 
	{
		private var bmd:BitmapData = new BitmapData(400, 200, false, 0x8080C0);
		private var brush:Sprite = new Sprite();
		public function Main():void 
		{
 
			brush.graphics.beginFill(0xC0C0C0);
			brush.graphics.drawCircle(0, 0, 10);
			brush.blendMode = BlendMode.ADD;
 
			addChild(new Bitmap(bmd));
			addChild(brush);
			stage.addEventListener(MouseEvent.MOUSE_MOVE, stage_mouseMove);
		}
 
		private function stage_mouseMove(e:MouseEvent):void 
		{
			brush.x = mouseX;
			brush.y = mouseY;
			bmd.draw(this);
		}
 
	}
}