The following color matrix filter will grayscale an image.

image-grayscale

private const matrix:Array = [
  		0.3, 0.59, 0.11, 0, 0,
		0.3, 0.59, 0.11, 0, 0,
		0.3, 0.59, 0.11, 0, 0,
		  0,    0,    0, 1, 0];
private const zero:Point = new Point();

grayscale.bitmapData.applyFilter(image.bitmapData, image.getRect(image), zero, new ColorMatrixFilter(matrix));