<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/08/25/setting-the-background-alpha-of-the-richtexteditor-text-area-in-flex/ -->
<mx:Application name="RichTextEditor_textArea_backgroundAlpha_test"
        xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            import mx.events.ColorPickerEvent;
            import mx.events.SliderEvent;

            private function colorPicker_change(evt:ColorPickerEvent):void {
                richTextEditor.textArea.setStyle("backgroundColor", evt.color);
            }

            private function slider_change(evt:SliderEvent):void {
                richTextEditor.textArea.setStyle("backgroundAlpha", evt.value);
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="backgroundColor:">
                <mx:ColorPicker id="colorPicker"
                        selectedColor="0xFFFFFF"
                        change="colorPicker_change(event);" />
            </mx:FormItem>
            <mx:FormItem label="backgroundAlpha:">
                <mx:HSlider id="slider"
                        minimum="0.0"
                        maximum="1.0"
                        value="1.0"
                        snapInterval="0.01"
                        tickInterval="0.1"
                        liveDragging="true"
                        change="slider_change(event);" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:RichTextEditor id="richTextEditor" />            

</mx:Application>