The following example shows you how you can create semi-transparent pop up menus using the Flex PopUpButton control by setting the backgroundAlpha, backgroundColor, and borderStyle styles on the pop up menu.

Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/01/30/creating-semi-transparent-popupbutton-pop-up-menus-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Style>
        PopUpButton {
            popUpStyleName: myCustomPopUpStyleName;
        }

        .myCustomPopUpStyleName {
            fontWeight: normal;
            textAlign: left;
            backgroundAlpha: 0.4;
            backgroundColor: white;
            borderStyle: solid;
        }
    </mx:Style>

    <mx:Script>
        <![CDATA[
            import mx.controls.Menu;

            private var menu:Menu;

            private function init():void {
                menu = new Menu();
                menu.labelField = "@label";
                menu.dataProvider = xmlList;
                popUpButton.popUp = menu;
                menu.width = popUpButton.width;
            }
        ]]>
    </mx:Script>

    <mx:XMLList id="xmlList">
        <node label="Alert" />
        <node label="Button" />
        <node label="ButtonBar" />
        <node label="CheckBox" />
        <node label="ColorPicker" />
        <node label="ComboBox" />
    </mx:XMLList>

    <mx:ApplicationControlBar dock="true">
        <mx:PopUpButton id="popUpButton"
                label="Please select an item..."
                openAlways="true"
                creationComplete="init();" />
    </mx:ApplicationControlBar>

    <mx:VBox backgroundAlpha="0.5"
            backgroundColor="haloSilver"
            width="100%"
            height="100%" />

</mx:Application>

View source is enabled in the following example.