Bootstrap 4 navbar-inverse doesn't have colour

In Bootstrap 4 you want to change the background class the nav tag (.bg-light or .bg-dark).

<nav class="navbar navbar-expand-md bg-dark">

Next you can use navbar-dark to change the color of the text to fit the darker contrast.

Navbar-inverse have property by default. It gives black color of navigation bar . Just change the .navbar-defaultclass into .navbar-inverse

<nav class="navbar navbar-inverse">

   In bootstrap 4 navbar-inverse is replace by navbar-dark. The way it works is, you specify light or dark (via .navbar-light or .navbar-dark)

<nav class="navbar navbar-expand-sm navbar-dark bg-primary">

use this :

<nav class="navbar navbar-expand-sm navbar-dark bg-dark">

Although the answer by @Asia-x3 helps a lot, but steel it does not change the forground color. You can get the same layout as the previouse navbar-inverse by doing the following modification to the <nav> tag class property and adding navbar-dark after bg-dark

<nav class="navbar navbar-expand-sm bg-dark navbar-dark">

Bootstrap-4 has dropped inverse classes. Instead you can use:

<nav class="navbar navbar-expand-lg navbar-light bg-light"> 

For more about Bootstrap 4, please visit: https://getbootstrap.com/docs/4.6/migration/

For converting your V3 code to V4 try: http://upgrade-bootstrap.bootply.com/