ellipse2.AddHandler(ToolTipService.ToolTipOpeningEvent,
    new RoutedEventHandler(whenToolTipOpens));
ellipse2.AddHandler(ToolTipService.ToolTipClosingEvent,
    new RoutedEventHandler(whenToolTipCloses));

void whenToolTipOpens(object sender, RoutedEventArgs e)
{
    Ellipse ell = new Ellipse();
    if (sender.GetType().FullName.Equals("System.Windows.Shapes.Ellipse"))
    {
        ell = (Ellipse)sender;
        ell.Fill = Brushes.Blue;
    }
    else if (sender.GetType().FullName.Equals(
                             "System.Windows.Controls.ToolTip"))
    {
        ToolTip t = (ToolTip)sender;
        Popup p = (Popup)t.Parent;
        ell = (Ellipse)p.PlacementTarget;
        ell.Fill = Brushes.Blue;
    }
}

void whenToolTipCloses(object sender, RoutedEventArgs e)
{
    Ellipse ell = new Ellipse();
    if (sender.GetType().FullName.Equals(
                      "System.Windows.Shapes.Ellipse"))
    {
        ell = (Ellipse)sender;
        ell.Fill = Brushes.Gray;
    }
    else if (sender.GetType().FullName.Equals(
                           "System.Windows.Controls.ToolTip"))
    {
        ToolTip t = (ToolTip)sender;
        Popup p = (Popup)t.Parent;
        ell = (Ellipse)p.PlacementTarget;
        ell.Fill = Brushes.Gray;
    }
}