<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/06/18/setting-an-opaque-background-on-a-radiobutton-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="opaqueBackground:">
                <mx:ColorPicker id="colorPicker" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:RadioButton id="radioButton"
            label="The quick brown fox jumped over the lazy dog"
            opaqueBackground="{colorPicker.selectedColor}" />

</mx:Application>