Laravel - Notifications - Error Messages

Some notifications inform users of errors, such as a failed invoice payment. You may indicate that a mail message is regarding an error by calling the error method when building your message. When using the error method on a mail message, the call to action button will be red instead of black:

     * Get the mail representation of the notification.
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Message
    public function toMail($notifiable)
        return (new MailMessage)
                    ->subject('Notification Subject')