Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/01/01/changing-the-preview-color-width-for-the-colorpicker-control-in-flex-3/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

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

            private function slider_change(evt:SliderEvent):void {
                var cssObj:CSSStyleDeclaration = StyleManager.getStyleDeclaration(".myCustomSwatchPanelStyleName");
                cssObj.setStyle("previewWidth", evt.value);

                colorPicker.open();
            }
        ]]>
    </mx:Script>

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

        .myCustomSwatchPanelStyleName {
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="previewWidth:">
                <mx:HSlider id="slider"
                        minimum="0"
                        maximum="145"
                        value="45"
                        snapInterval="1"
                        tickInterval="5"
                        liveDragging="true"
                        dataTipPrecision="0"
                        change="slider_change(event);" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:ColorPicker id="colorPicker" />

</mx:Application>

View source is enabled in the following example.

You can also set the previewWidth style directly in an external .CSS file or <mx:Style /> block using something similar to the following snippet:

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

    .myCustomSwatchPanelStyleName {
        previewWidth: 145;
    }
</mx:Style>