Sorry, I didn’t even see the colored line. I do that with a border on the inside-nav element when I need a border color. I use either Simple CSS plugin or a child-theme style.css to add the bits of CSS I want. I did this to the site http://www.realfoodtips.com/ to add the lighter grey on the top of the nav bar. Here’s the CSS for that one.
border-top: solid 4px #ccc;