When validating a field manually from my form a error is displayed but the field doesn't get red. For the first parameter i used the name of the input element.

if($form_state['values']['panes']['webform_nid31'][0]['contact_person'] == '') {
    form_set_error('panes[webform_nid31][0][contact_person]', t('You must select a name for this group of settings.'));
  }