You can't change that color via the public object model...the following code sample demonstrates how to do it using the IUIElementCreationFilter interface: 

AppearanceData appearance = new AppearanceData();
appearance.BackColor = Color.Red;
appearance.BackColor2 = Color.Pink;
this.listView.CreationFilter = new GroupDividerCreationFilter( appearance );
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#region GroupDividerCreationFilter
/// <summary>
/// IUIElementCreationFilter implementation which Handles the display of
/// checkboxes for UltraListViewItems in Thumbnails mode.
/// </summary>
public class GroupDividerCreationFilter : IUIElementCreationFilter
{
    private AppearanceData appearance;

    /// <summary>
    /// Creates a new instance of the GroupDividerCreationFilter class.
    /// </summary>
    /// <param name="appearance">Defines the BackColor, BackColor2, and BackGradientStyle properties for the divider element.</param>
    public GroupDividerCreationFilter( AppearanceData appearance )
    {
        this.appearance = appearance;
    }

    #region IUIElementCreationFilter Members

    void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
    {
        if ( parent is UltraListViewGroupUIElement )
        {
            UltraListViewGroupHeaderLineUIElement dividerElement =
                parent.GetDescendant( typeof(UltraListViewGroupHeaderLineUIElement) )
                as UltraListViewGroupHeaderLineUIElement;

            if ( dividerElement != null )
            {
                Rectangle rect = dividerElement.Rect;
                UIElementsCollection childElements = parent.ChildElements;
                childElements.Remove( dividerElement );

                GroupDividerUIElement newElement = new GroupDividerUIElement(parent as UltraListViewGroupUIElement, this.appearance);
                newElement.Rect = rect;
                childElements.Add( newElement );
            }
        }
    }

    bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
    {
        //  Return false to signify that the elements
        //  should be added as they normally are.
       return false;
    }

    #endregion IUIElementCreationFilter Members

    #region GroupDividerUIElement class
    public class GroupDividerUIElement : UltraListViewGroupHeaderLineUIElement
    {
        private AppearanceData appearance;

        /// <summary>
        /// Creates a new instance of the GroupDividerUIElement class.
        /// </summary>
        public GroupDividerUIElement( UltraListViewGroupUIElement parent, AppearanceData appearance ) : base( parent )
        {
            this.appearance = appearance;
        }

        protected override void InitAppearance(ref AppearanceData appearance, ref AppearancePropFlags requestedProps)
        {
            AppearanceData.Merge( ref appearance, ref this.appearance, ref requestedProps );
            base.InitAppearance(ref appearance, ref requestedProps);
        }
    }
    #endregion GroupDividerUIElement class
}
#endregion GroupDividerCreationFilter