Did you already sow a new version of Marquee Tool? In that story is a sample of changing marquee appearance. Now you can easily change color/opacity of picture marquee. Hit Alt and then click up/down buttons. This is a shortest way to increase/decrease marquee opacity on the fly. Also you can use setColor, setOpacity methods. All developers welcome
MarqueeTool v.2. How to change shroud Color and Opacity?