Drupal 7. Preproccess colorbox images.

template.php

/**
 * Implements template_preprocess_field
 */
function THEME_preprocess_field(&$vars) {
  // Display first image in colorbox gallery preview.
  if (isset($vars['element']['#field_name']) && $vars['element']['#field_name'] == 'field_product_image') {
    if (isset($vars['items'][0]['#theme']) && ($vars['items'][0]['#theme'] == 'colorbox_image_formatter')) {
      $first_photo_copy = $vars['items'][0];

      // Disable copy image in gallery, use only for preview.
      $first_photo_copy['#display_settings']['colorbox_gallery'] = 'none';

      // Add copy image to colorbox gallery.
      array_push($vars['items'], $first_photo_copy);
    }
  }
}