You can add darken on scroll feature to the Bootstrap navbar, just add data-navbar-darken-on-scroll attribute to the .navbar element. By default it will take the bg-dark color code to darken the navbar.

<nav class="navbar navbar-expand-lg navbar-dark" data-navbar-darken-on-scroll="data-navbar-darken-on-scroll">

  <!-- Navbar content -->
</nav>

You can also change the background color of the navbar by using any of the color from the list: primary, secondary, success, info, warning, danger, light, dark, black, 1100, 1000, 900, 800, 700, 600, 500, 400, 300, 200, 100, white like as follows: See all colors in Falcon

<nav class="navbar navbar-expand-lg navbar-dark" data-navbar-darken-on-scroll="primary">

  <!-- Navbar content -->
</nav>