Answers for "on scroll javascript change background color"

change scrollbar color

.scrollable-element {
  scrollbar-color: red yellow; /* red is for the thumb and yellow is for the track */
}

javascript on scroll change nav color

$(function () {
  $(document).scroll(function () {
    var $nav = $(".navbar-fixed-top");
    $nav.toggleClass('scrolled', $(this).scrollTop() > $nav.height());
  });
});