如何在使用ajax的codeigniter中使验证错误的字段变红

<form id="myForm">
  <input type="text" name="username" id="username">
  <input type="password" name="password" id="password">
  <button type="button" onclick="submitForm()">Submit</button>
</form>

<style>
  .error {
    background-color: red;
  }
</style>

<script>
  function submitForm() {
    var formData = $('#myForm').serialize();
    $.ajax({
      url: 'your_validation_url',
      type: 'POST',
      data: formData,
      success: function(response) {
        if (response.error) {
          $.each(response.error, function(field, message) {
            $('#' + field).addClass('error');
          });
        }
      }
    });
  }
</script>

public function validateForm() {
  $this->load->library('form_validation');
  $this->form_validation->set_rules('username', 'Username', 'required');
  $this->form_validation->set_rules('password', 'Password', 'required');

  if ($this->form_validation->run() == FALSE) {
    $errors = array();
    $errors['username'] = form_error('username');
    $errors['password'] = form_error('password');
    echo json_encode(array('error' => $errors));
  } else {
    // 验证通过，执行其他操作
  }
}