To change the background color of Sub Menu:
.genesis-nav-menu .sub-menu a:hover,
.genesis-nav-menu .sub-menu li.current-menu-item > a {
background-color: #ff0000;
color: #fff;
}
Add that code to your CSS. Just so you know, you can change the main default color of the Theme (orange) to whatever color you want in the Admin Customize page. But this changes it all in one. If you want to have a certain section of your site separate, then you would have to add something like the code listed above as an example.
I added a red background in the code so you can test to see if it works.