diff -u b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
@@ -108,16 +108,18 @@
    * @param string $filename
    *   The file name of the resulting screenshot. If using the default phantomjs
    *   driver then this should be a JPG filename.
+   * @param string $background_color
+   *   (optional) Background color name.
    *
    * @throws \Behat\Mink\Exception\UnsupportedDriverActionException
    *   When operation not supported by the driver.
    * @throws \Behat\Mink\Exception\DriverException
    *   When the operation cannot be done.
    */
-  protected function createScreenshot($filename, $background = 'white') {
+  protected function createScreenshot($filename, $background_color = 'white') {
     $session = $this->getSession();
     if (!empty($background)) {
-      $session->executeScript("document.body.style.backgroundColor = '" . $background . "';");
+      $session->executeScript("document.body.style.backgroundColor = '" . $background_color . "';");
     }
     $image = $session->getScreenshot();
     file_put_contents($filename, $image);