Wordpress menu CSS Classes is not working

Wordpress menu CSS Classes is not working

I want to change the color of one of the tabs of the menu bar in Wordpress
using menu CSS Classes, I added the class name in my style file, but when
I add the class name in the CSS Classes in the Admin->Menu page it's not
working. that's the class name in the style file it's very simple.
.red{
color: #ff0000;
}