Issue #2196815 by kirby14: Convert all calls & docs references to...

Issue #2196815 by kirby14: Convert all calls & docs references to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in color module.

core/modules/color/color.module

@@ -5,6 +5,7 @@
  */
 
 use Drupal\Core\Asset\CssOptimizer;
+use Drupal\Component\Utility\String;
 
 /**
  * Implements hook_help().
@@ -214,7 +215,7 @@ function color_scheme_form($complete_form, &$form_state, $theme) {
     if (isset($names[$name])) {
       $form['palette'][$name] = array(
         '#type' => 'textfield',
-        '#title' => check_plain($names[$name]),
+        '#title' => String::checkPlain($names[$name]),
         '#value_callback' => 'color_palette_color_value',
         '#default_value' => $value,
         '#size' => 8,