void myListBox_SelectionChanged(object sender, SelectionChangedEventArgs args)
{

    BrushConverter converter = new BrushConverter();

    // Show Rectangles that are the selected colors.
    foreach (string color in args.AddedItems)
    {
        if (GetRectangle(color) == null)
        {
            Rectangle aRect = new Rectangle();
            aRect.Fill = (Brush) converter.ConvertFrom(color);
            aRect.Tag = color;
            rectanglesPanel.Children.Add(aRect);
        }
    }

    // Remove the Rectangles that are the unselected colors.
    foreach (string color in args.RemovedItems)
    {
        FrameworkElement removedItem = GetRectangle(color);
        if (removedItem != null)
        {
            rectanglesPanel.Children.Remove(removedItem);
        }
    }
}

FrameworkElement GetRectangle(string color)
{
    foreach (FrameworkElement rect in rectanglesPanel.Children)
    {
        if (rect.Tag.ToString() == color)
            return rect;
    }

    return null;
}