You should learn to use a web debugging tool like Firebug (a free Firefox extension) or Chrome Developer Tools, which comes built-in with Chrome. I used Chrome DevTools to look at your site and figure out what selector to use. All you need to do is right-click on an element and select Inspect element from the pop-up menu and you’ll see all of the CSS rules which affect that element.
Other selectors that you’ll need to style:
#site-navigation .dropdown-menu > .current-menu-item > a
As you can tell from the wording, this targets the color of the current menu item, which is the menu item that corresponds to the page that you are viewing. If you are on the Home page, then this would affect the color of the Home menu item.
#site-navigation .dropdown-menu ul a
As Kristen indicated earlier, this targets the drop down menu items.
If you’re looking for a good CSS tutorial, I like this one.