It looks like this is not directly supported, but you can accomplish it with the UIElements in the color picker drop down. Here is the code you need (make sure to hook the BeforeToolDropdown and AfterToolCloseup events of the toolbars manager):

private UltraControlBase hookedControl;

private void HookControl( UltraControlBase control )
{
 this.UnhookControl();

 if ( control == null )
  return;

 control.MouseEnterElement += new UIElementEventHandler( this.control_MouseEnterElement );
 control.MouseLeaveElement += new UIElementEventHandler( this.control_MouseLeaveElement );
 this.hookedControl = control;
}

private void UnhookControl()
{
 if ( this.hookedControl == null )
  return;

 this.hookedControl.MouseEnterElement -= new UIElementEventHandler( this.control_MouseEnterElement );
 this.hookedControl.MouseLeaveElement -= new UIElementEventHandler( this.control_MouseLeaveElement );
 this.hookedControl = null;
}

private void ultraToolbarsManager1_BeforeToolDropdown( object sender, BeforeToolDropdownEventArgs e )
{
 if ( e.Tool is PopupColorPickerTool )
  this.HookControl( e.Tool.Control as UltraControlBase );
}

private void ultraToolbarsManager1_AfterToolCloseup( object sender, ToolDropdownEventArgs e )
{
 this.UnhookControl();
}

private void control_MouseEnterElement( object sender, UIElementEventArgs e )
{
 CustomColorBoxHotTrackingButtonUIElement colorBox = e.Element as CustomColorBoxHotTrackingButtonUIElement;

 if ( colorBox != null )
 {
  CustomColor customColor = (CustomColor)colorBox.GetContext();
  Color color = customColor.Color;

  // The mouse has entered a color box, show the live preview with the color.
 }
}

private void control_MouseLeaveElement( object sender, UIElementEventArgs e )
{
 if ( e.Element is CustomColorBoxHotTrackingButtonUIElement )
 {
  // The mouse has left the color box, clear the live preview
 }
}