code: protected void updateBorder (boolean modified)
      {
          if (modified) {
              setBorder(BorderFactory.createMatteBorder(2, 2, 2, 2, Color.red));
          } else {
              setBorder(BorderFactory.createEmptyBorder(2, 2, 2, 2));
          }
      }
docs: Sets the appropriate border on this field editor based on whether
  or not the field is modified.