Code

void giveMeAllConnectedBoxes(Box box, BoxCollection connectedBoxes)
{
  if (connectedBoxes.Contains(box))
    return;

  connectedBoxes.Add(box);

  foreach (Arrow arrow in box.OutgoingArrows)
  {
    Box otherBox = arrow.Destination as Box;
    if (otherBox != null)
     giveMeAllConnectedBoxes(otherBox, connectedBoxes);
  }

  foreach (Arrow arrow in box.IncomingArrows)
  {
    Box otherBox = arrow.Origin as Box;
    if (otherBox != null)
     giveMeAllConnectedBoxes(otherBox, connectedBoxes);
  }
}

private void button3_Click(object sender, System.EventArgs e)
{
  Box box = fc.ActiveObject as Box;
  if (box != null)
  {
    BoxCollection connected = new BoxCollection();
    giveMeAllConnectedBoxes(box, connected);
    foreach (Box cb in connected)
     cb.FillColor = Color.Red;
  }
}