Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/11/24/setting-a-columnchart-controls-item-roll-over-color-and-item-selection-color-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.charts.chartClasses.IAxis;

            private function linearAxis_labelFunc(item:Object, prevValue:Object, axis:IAxis):String {
                return numberFormatter.format(item);
            }
        ]]>
    </mx:Script>

    <mx:NumberFormatter id="numberFormatter" precision="3" />

    <mx:ArrayCollection id="arrColl">
        <mx:source>
            <mx:Array>
                <mx:Object name="R Winn" obp=".353" slg=".445" avg=".300" />
                <mx:Object name="P Feliz" obp=".290" slg=".418" avg=".253" />
                <mx:Object name="O Vizquel" obp=".305" slg=".316" avg=".246" />
                <mx:Object name="B Molina" obp=".298" slg=".433" avg=".276" />
                <mx:Object name="R Durham" obp=".295" slg=".343" avg=".218" />
            </mx:Array>
        </mx:source>
    </mx:ArrayCollection>

    <mx:ColumnChart id="columnChart"
            showDataTips="true"
             dataProvider="{arrColl}"
             selectionMode="single"
             itemRollOverColor="haloSilver"
             itemSelectionColor="haloBlue"
             width="100%"
             height="100%">

        <mx:horizontalAxis>
            <mx:CategoryAxis id="ca"
                    categoryField="name" />
        </mx:horizontalAxis>

        <mx:verticalAxis>
            <mx:LinearAxis baseAtZero="false"
                    minimum="0.200"
                    maximum="0.310"
                    labelFunction="linearAxis_labelFunc" />
        </mx:verticalAxis>

        <mx:horizontalAxisRenderers>
            <mx:AxisRenderer axis="{ca}"
                    canDropLabels="false" />
        </mx:horizontalAxisRenderers>

        <mx:series>
            <mx:ColumnSeries id="columnSeries"
                    xField="name"
                    yField="avg"
                    displayName="avg" />
        </mx:series>

        <mx:seriesFilters>
            <mx:Array />
        </mx:seriesFilters>

    </mx:ColumnChart>

</mx:Application>

View source is enabled in the following example.

You can also set the itemSelectionColor and itemRollOverColor styles using CSS, as seen in the following snippet:

<mx:Style>
    ColumnChart {
        itemRollOverColor: haloSilver;
        itemSelectionColor: haloBlue;
    }
</mx:Style>

Or, you can set the styles using ActionScript, as seen in the following snippet:

columnChart.setStyle("itemRollOverColor", "haloSilver");
columnChart.setStyle("itemSelectionColor", "haloBlue");