Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/12/29/changing-the-swatch-highlight-size-for-a-colorpicker-control-in-flex-3/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white"
        creationComplete="init();">

    <mx:Style>
        ColorPicker {
            swatchPanelStyleName: myCustomSwatchPanelStyleName;
        }

        .myCustomSwatchPanelStyleName {
            /* border */
            swatchBorderColor: white;
            swatchBorderSize: 1;
            /* highlight */
            swatchHighlightColor: red;
            swatchHighlightSize: 1;
        }
    </mx:Style>

    <mx:Script>
        <![CDATA[
            import mx.events.SliderEvent;

            private var cssObj:CSSStyleDeclaration;

            private function init():void {
                cssObj = StyleManager.getStyleDeclaration(".myCustomSwatchPanelStyleName");
                colorPicker.open();
            }

            private function borderSlider_change(evt:SliderEvent):void {
                cssObj.setStyle("swatchBorderSize", evt.value);
                colorPicker.open();
            }

            private function highlightSlider_change(evt:SliderEvent):void {
                cssObj.setStyle("swatchHighlightSize", evt.value);
                colorPicker.open();
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="swatchBorderSize:">
                <mx:HSlider id="borderSlider"
                        minimum="0"
                        maximum="5"
                        value="1"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="1"
                        change="borderSlider_change(event);" />
            </mx:FormItem>
            <mx:FormItem label="swatchHighlightSize:">
                <mx:HSlider id="highlightSlider"
                        minimum="0"
                        maximum="5"
                        value="1"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="1"
                        change="highlightSlider_change(event);" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:ColorPicker id="colorPicker" />

</mx:Application>

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