OK, thank you for the clarification.
You just need a slight modification to the above CSS. Here are the first two rules that you’ll need, the first rule is for the first menu item, the second rule is for the second menu item:
#site-navigation li:nth-child(1) a:hover {
border-top: #FF0000 solid 4px;
}
#site-navigation li:nth-child(2) a:hover {
border-top: #0000FF solid 4px;
}
The key is using what’s called the nth-child(n) pseudo-selector. The li:nth-child(1) selects the first menu item, the li:nth-child(2) selects the second menu item. So just repeat each rule for as many menu items that you have, i.e., add a rule for li:nth-child(3) to target the third menu item, and so on.
Then for each rule, change the hex value for the border-top (the line that goes across the top of the menu item). In the example above, the first menu item gets a color of red, the second menu item gets a color of blue.