The following example formats a column in a Flex DataGrid and uses a custom item renderer to color the text red in a cell if a price is below $0. If the item is greater than $0, the test is displayed in black. The price column is also formatted using a custom label function, which uses a CurrencyFormatter, and finally, the data grid column uses a custom sort function to properly sort numeric columns.

Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/08/20/formatting-a-flex-datagrid-control-using-a-custom-item-renderer/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.controls.dataGridClasses.DataGridColumn;
            import mx.utils.ObjectUtil;

            private function price_labelFunc(item:Object, column:DataGridColumn):String {
                return currencyFormatter.format(item.@price);
            }

            private function price_sortCompareFunc(itemA:Object, itemB:Object):int {
                return ObjectUtil.numericCompare(itemA.@price, itemB.@price);
            }
        ]]>
    </mx:Script>

    <mx:XML id="itemsXML">
        <items>
            <item name="Item 1" price="1.32" />
            <item name="Item 2" price="-12.23" />
            <item name="Item 3" price="4.96" />
            <item name="Item 4" price="-0.94" />
        </items>
    </mx:XML>

    <mx:Style>
        .centered {
            text-align: center;
        }
    </mx:Style>

    <mx:CurrencyFormatter id="currencyFormatter"
            precision="2"
            useNegativeSign="false" />

    <mx:DataGrid id="dataGrid" dataProvider="{itemsXML.item}">
        <mx:columns>
            <mx:DataGridColumn dataField="@name"
                    headerText="Name:"
                    headerStyleName="centered" />

            <mx:DataGridColumn dataField="@price"
                    headerText="Price:"
                    textAlign="right"
                    headerStyleName="centered"
                    labelFunction="price_labelFunc"
                    sortCompareFunction="price_sortCompareFunc"
                    itemRenderer="PriceLabel" />
        </mx:columns>
    </mx:DataGrid>

</mx:Application>

PriceLabel.as

package {
    import mx.controls.Label;
    import mx.controls.listClasses.*;

    public class PriceLabel extends Label {

        private const POSITIVE_COLOR:uint = 0x000000; // Black
        private const NEGATIVE_COLOR:uint = 0xFF0000; // Red 

        override protected function updateDisplayList(unscaledWidth:Number, unscaledHeight:Number):void {
            super.updateDisplayList(unscaledWidth, unscaledHeight);

            /* Set the font color based on the item price. */
            setStyle("color", (data.@price <= 0) ? NEGATIVE_COLOR : POSITIVE_COLOR);
        }
    }
}

View source is enabled in the following example.