I try the one of your approach, it works well in my computer. Below is my code:

 public void AddContextMenu()
  {
   ContextMenu mnuContextMenu = new ContextMenu();
   mnuContextMenu.MenuItems.Add("&Red LED", new EventHandler(SetDisplayRed));
   mnuContextMenu.MenuItems.Add("&Blue LED", new EventHandler(SetDisplayBlue));
   mnuContextMenu.MenuItems.Add("&LCD", new EventHandler(SetDisplayLCD));
   textBox1.ContextMenu = mnuContextMenu;
   this.ContextMenu = mnuContextMenu;
  }

  private void SetDisplayRed(object sender, EventArgs e)
  {

   textBox1.BackColor = Color.Maroon;
   textBox1.ForeColor = Color.OrangeRed;
  }

  private void SetDisplayBlue(object sender, EventArgs e)
  {
   textBox1.BackColor = Color.Red;
   textBox1.ForeColor = Color.Gray;
  }

  private void SetDisplayLCD(object sender, EventArgs e)
  {
   textBox1.BackColor = Color.Beige;
   textBox1.ForeColor = Color.Blue;
  }

>When I right-click on the textbox I get a menu with Undo/Cut/Copy/Paste/Select All.  And when I right-click on the form I get nothing

Have you already added your "AddContextMenu()" method into the contruction method of your mainform?

public MainForm()
  {
   InitializeComponent();

   AddContextMenu();
  }