// calculate x distance
function distance(x1:Number, y1:Number, x2:Number, y2:Number):Number {
var x2x1:Number = x2 - x1;
var y2y1:Number = y2 - y1;
return Math.sqrt(x2x1 * x2x1 + y2y1 * y2y1);
}

var colortr: ColorTransform = new ColorTransform();

//check overlap in every frame
function checkdist(e: Event):void {
var color:Number = 0xFFFFFF;

if (distance(black.x, black.y, blu.x, blu.y)< black.width/2 + blu.width/2) {//r1+r2
color = 0x3399CC;
}

colortr.color = color;
box.transform.colorTransform = colortr;//box has a prop, that has a prop
}

addEventListener(Event.ENTER_FRAME, checkdist);