How to show icon in combo box's selected item?

preview swf:

the code below is able to show icons in the option items,
but when i select an item from the option,
the icon does not appear in the selected item.
How do I make that icon appear inside selected item?

var blue:Sprite = new blue_mc();
var yellow:Sprite = new yellow_mc();
var green:Sprite = new green_mc();
var red:Sprite = new red_mc();

//cb.height = 35;
//cb.width = 130;
cb.dropdown.rowHeight = 35;
cb.addItem ({label:"blue",icon:blue});
cb.addItem ({label:"yellow",icon:yellow});
cb.addItem ({label:"green",icon:green});
cb.addItem ({label:"red",icon:red});