A few things

1) Did you link this css to your html with the link tag

2) In css classes are preceded with a dot and id's with a # symbol

example of a class here would be .selected

3) Another when selecting pseudo classes like hover, visited etc. we dont use a dot but a colon : instead ...

for e.g. you have written nav a, nav a.visited ... this is incorrect, same goes for nav a.hover

it should have been nav a, nav a:visited ... and nav a:hover ... this will work fine

4) I would suggest to indent your css code properly. I know its a boring task but it helps you visualize mistakes quicker

I assume logo is an id . So it should be

#logo {
    text-align: center; 
    margin:0;
}

nav a {
    background: #599a68;
} 

nav a, nav a:visited {
   color: #fff;
} 

nav a.selected , nav a:hover {
     color: #32673f;
}

Hope this helps.

If you do have doubts ask again

You probably need to use pseudo-classes (:) instead of classes a:link, a:visited, a:hover, a:active

There is no 'selected' pseudo class so you will use that as a standard class, but make sure it's in your HTML that way.

A link to read about pseudo classes: http://www.w3schools.com/css/css_pseudo_classes.asp

a few things logo is not a semantic tag the code should be

       #logo  {
      text-align: center;
      margin:0;
      //or

. logo depending whether it is class or not }

and it should be css

 nav a:hover{}
 //and
 nav a:selected{}