To change the background color of the top navigation bar go to editor under appearance. From the right hand corner click on style.css
You will then see the style.css files. Scroll down to and look for navigation as as shown on the image below. Replace the color code with your color code of your choice. Visit this link where you can get color codes for HTML: http://html-color-codes.info/
Next, you have to continue to scroll down and look for the code as shown below. Replace the background color with the same code you have used in the in the above image.
After you have done your changes please don't forget to click the update file button to save your changes.
If the changes doesn't show, add !important after the color code.