You need to dynamically set the color of a MovieClip.
Use the colorTransform property to set the color.
var clip:MovieClip = attachMovie("circle_clip", "circle_clip", 1);
clip._x = 100;
clip._y = 100;
function onStageClick():Void
{
var c:Color = new Color(clip);
c.setRGB(Math.random() * 0xFFFFFF);
}
onMouseUp = onStageClick;
var clip:MovieClip = new circle_clip();
clip.x = 100;
clip.y = 100;
addChild(clip);
function onStageClick(event:MouseEvent):void
{
var c:ColorTransform = new ColorTransform();
c.color = (Math.random() * 0xFFFFFF);
clip.transform.colorTransform = c;
}
stage.addEventListener(MouseEvent.CLICK, onStageClick);
Note: Math.random * 0xFFFFFF creates a random color.