Full code after the jump.

The following example shows how you can set fill colors in a Flex PieChart by specifying a custom fill function. In this example the fill function calculates the fill color based on the percentage value of each item. The larger the wedge, the brigher the fill color.

View MXML

<?xml version="1.0"?>
<!-- http://blog.flexexamples.com/2007/11/07/creating-custom-fills-in-a-flex-piechart-control-using-the-fillfunction-property/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.charts.series.items.PieSeriesItem;
            import mx.graphics.IFill;
            import mx.charts.ChartItem;
            import mx.graphics.SolidColor;

            private function pieSeries_fillFunc(item:ChartItem, index:Number):IFill {
                var curItem:PieSeriesItem = PieSeriesItem(item);
                /* Convert to a number between 0 and 1. */
                var pct:Number = curItem.percentValue / 100;
                return new SolidColor(0x0000FF * pct, 1.0);
            }
        ]]>
    </mx:Script>

    <mx:XML id="dp">
        <products>
            <product label="Product 1" data="3" />
            <product label="Product 2" data="1" />
            <product label="Product 3" data="4" />
            <product label="Product 4" data="1" />
            <product label="Product 5" data="5" />
            <product label="Product 6" data="9" />
        </products>
    </mx:XML>

    <mx:PieChart id="chart"
            height="100%"
            width="100%"
            showDataTips="true"
            dataProvider="{dp.product}">
        <mx:series>
            <mx:PieSeries id="series"
                    nameField="@label"
                    field="@data"
                    fillFunction="pieSeries_fillFunc"
                    filters="[]" />
        </mx:series>
    </mx:PieChart>

</mx:Application>

View source is enabled in the following example.