gistfile1.txt

/**
 * Implements hook_url_outbound_alter().
 */
function MODULE_url_outbound_alter(&$path, &$options, $original_path) {
  preg_match('@^search/(.+)@', $path, $matches);
    // Only tack on a color nid facet if there isn't one already selected.
    if (FALSE === strpos($matches[1],'/')) {
      // The alias path will be the letter 'c'
      $path .= '/c/' . color_selector($_COOKIE['color']);
    }
}

/**
 * Activate the pretty paths by default on all searchvers.
 *
 * @param array $searcher_info
 *   Searcher info.
 */
function mercy_solr_facetapi_searcher_info_alter(array &$searcher_info) {
  foreach ($searcher_info as &$info) {
    // Activate pretty paths on all searchers.
    $info['url processor'] = 'pretty_paths';
    $info['facetapi pretty paths coder'] = 'default';
  }
}

Add a line to the global settings section of your MODULE_facetapi_defaults.inc
'pretty_paths_alias' => 'c',