B/W image to color tween, how?

programming flash
1

my problem is simple: i want an image to start in black and white, and slowly change to the original color image. How can i do this?
I found a way to make the image in b/w using the ColorMatrixFilter class in AS3, the code would be like this:

var gscale:ColorMatrixFilter=new ColorMatrixFilter([.2127,.7152,.0722,0,0,
                                                    .2127,.7152,.0722,0,0,
                                                    .2127,.7152,.0722,0,0,
                                                    0,0,0,1,0]);

img_mc.filters=[gscale];

the final color matrix filter should look like this:

var gscale:ColorMatrixFilter=new ColorMatrixFilter([1,0,0,0,0,
                                                    0,1,0,0,0,
                                                    0,0,1,0,0,
                                                    0,0,0,1,0]);

img_mc.filters=[gscale];

how can i tween this values?? as far as i know, the tween class only accepts properties like alpha, rotation, etc.
Any ideas?