<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/06/09/setting-a-border-color-on-the-combobox-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Style>
        ComboBox {
            borderColor: red;
        }
    </mx:Style>

    <mx:Array id="arr">
        <mx:Object label="One" />
        <mx:Object label="Two" />
        <mx:Object label="Three" />
        <mx:Object label="Four" />
        <mx:Object label="Five" />
        <mx:Object label="Six" />
    </mx:Array>

    <mx:ComboBox id="comboBox"
            dataProvider="{arr}" />

</mx:Application>