ItemRenderer for TreeColumn on AdvancedDataGrid in Flex

seems to work fine I've had a similar issue in the past when using HierarchicalData. The way I got around it was to use an AdvancedDataGridRendererProvider.
Here's some sample code:

<mx:AdvancedDataGrid width="100%" height="100%" 
    id="topAccountsGrid" 
    backgroundColor="#ffffff">
    <mx:dataProvider>
        <mx:HierarchicalData source="{filteredList}"
           childrenField="children" />
    </mx:dataProvider>
    <mx:columns>
       <mx:AdvancedDataGridColumn id="colRank" headerText="Rank" dataField="Rank__c" width="60" />
        <mx:AdvancedDataGridColumn id="list_name" headerText="Name" dataField="Name" />
        <mx:AdvancedDataGridColumn id="colPrevRank" headerText="Previous Rank" dataField="Previous_Rank__c" />
        <mx:AdvancedDataGridColumn id="colType" headerText="Type" dataField="Type" />
        <mx:AdvancedDataGridColumn id="colContacts" headerText="# Contacts" dataField="Contacts__c" />
        <mx:AdvancedDataGridColumn id="colDeals" headerText="# Deals" dataField="Deals__c" />
    </mx:columns>
    <mx:rendererProviders>
      <mx:AdvancedDataGridRendererProvider column="{colRank}" depth="1" dataField="Rank__c" renderer="AdvancedDataGridRankCRenderer" />
    </mx:rendererProviders>
</mx:AdvancedDataGrid>

<?xml version="1.0" encoding="utf-8"?>
<mx:HBox xmlns:fx="http://ns.adobe.com/mxml/2009" 
             xmlns:s="library://ns.adobe.com/flex/spark" 
             xmlns:mx="library://ns.adobe.com/flex/mx" 
             backgroundColor="{SetBackgroundColor(data)}" 
             paddingLeft="2" paddingRight="2" paddingTop="2"
             horizontalScrollPolicy="off"
             verticalScrollPolicy="off"
             height="22">
    <fx:Script>
        <![CDATA[
            [Bindable] private var bgColor:uint = 0xD6E5FF;

            private function SetBackgroundColor(obj:Object):uint
            {
                var returnColor:uint = 0xFF5050;

                if (obj["Rank__c"] != null)
                {
                    switch (obj["Rank__c"].toString().toUpperCase())
                    {
                        case "0":
                            returnColor = 0xFF5050;
                            break;
                        case "50":
                            returnColor = 0xFFFF99;
                            break;
                        case "100":
                            returnColor = 0x66FF66;
                            break;
                        default:
                            returnColor = 0xFF5050;
                                                    break;
                    }
                }

                return returnColor;
            }

            override public function set data(value:Object):void
            {
                super.data = value;
                rankLabel.text = value["Rank__c"].toString();               
                validateDisplayList();
            }
        ]]>
    </fx:Script>
    <mx:Label id="rankLabel" />
</mx:HBox>

like below fixes the issue The space between the dateInput and the icon is normally transparent. I'm not sure why mx:DataGrid hides the rendered text and the mx:AdvancedDataGrid doesn't.
Anyway, there's an easy solution to your problem. Just paint the itemEditors background in the color you like. The following should work.

<mx:itemEditor>
    <fx:Component>
        <mx:DateField yearNavigationEnabled="true" formatString="DD/MM/YY">
            <fx:Script>
                <![CDATA[
                    protected override function updateDisplayList(unscaledWidth:Number, unscaledHeight:Number):void
                    {
                        graphics.beginFill(0xFFFFFF); // white
                        graphics.drawRect(0, 0, unscaledWidth, unscaledHeight);
                        graphics.endFill();

                        super.updateDisplayList(unscaledWidth, unscaledHeight);
                    }
                ]]>
            </fx:Script>
        </mx:DateField>
    </fx:Component>
</mx:itemEditor>

seems to work fine You need to set the groupItemRenderer which is used to render the treeColumn (the column that displays the icons, which per default is the first column).

should help you out you can expand/collapse a specific item simply giving this statement to your adg:

adg.expandItem(<your_item>, !adg.isItemOpen(<your_item>))