I agree with Daniel : gtween, tweener and all the others does the work with ease.
If you want to custimize it you can get great use of getters and setters though (this is often used for more complex situations but may be interesting) :
private var _brightness:Number = 1;
public function set brightness(value:Number):void
{
_brightness = value
this.colorTransform = new ColorTransform(value,value,value);
}
public function get brightness():Number
{
return _brightness;
}
Examples :
Simple :
brightness = 0;
Random linear :
brightness -= Math.random() * .02;
Interactive and eased :
brightness += ((stage.mouseY / stage.stageHeight)-brightness) * .9;
etc.