Accessibility for screen readers

Workaround with javascript:

Drupal.behaviors.colorboxAccessibility = {
    attach: function (context, settings) {
      // Attach after Colorbox is loaded and opened
      $(document).on('cbox_open', function () {
        // Add ARIA labels to Colorbox buttons
        $('#cboxPrevious').attr('aria-label', Drupal.t('Previous'));
        $('#cboxNext').attr('aria-label', Drupal.t('Next'));
        $('#cboxSlideshow').attr('aria-label', Drupal.t('Start slideshow'));
        $('#cboxClose').attr('aria-label', Drupal.t('Close'));
      });
    }
  };