The following code is useful when using links with the .colorbox-load class following the pattern "http://www.example.com/path?width=500&height=500&iframe=true". Only the page content region is displayed and everything else gets hidden

template.php

<?php
/**
 * Implements hook_process_page().
 */
function MYTHEME_process_page(&$variables) {
  if (!empty($_GET['iframe'])) {
    foreach (element_children($variables['page']) as $index) {
      if ($index == 'content') {
        continue;
      }
      $variables['page'][$index]['#access'] = FALSE;
    }
    $variables['logo'] = '';
    $variables['main_menu'] = array();
    $variables['secondary_menu'] = array();
    $variables['tabs'] = array();
    $variables['title'] = '';
  }
}