The AS3 API gives pretty good documentation on how to use the treshhold value. You can find it here. Their example actually checks for a certain range of colors. I've modified their example to work for your problem. I haven't tested it, so it might need some tweaking.
var bmd2:BitmapData = new BitmapData(200, 200, true, 0xFFCCCCCC);
var pt:Point = new Point(0, 0);
var rect:Rectangle = new Rectangle(0, 0, 200, 200);
var threshold:uint = 0x00A9A9A9; //Dark Grey
var color:uint = 0x00000000; //Replacement color (white)
var maskColor:uint = 0xFFFFFFFF; //What channels to affect (this is the default).
bmd2.threshold(bmd1, rect, pt, ">", threshold, color, maskColor, true);
Another option would be to use a double for loop that iterates over all pixels and takes a certain action based on the value of the pixel.
for(var y:int = 0; y < height; y++){
for(var x:int = 0; x < width; x++){
var currentPixel:uint = image.getPixel( x, y );
if(currentPixel != color){
image.setPixel( destPoint.x + j, destPoint.y + i, currentPixel );
}
}
}