This should already exist in your css. Take a look around line 241 (if you’re using the default skin) and you’ll see this code –
.genesis-nav-menu li a:hover,
.genesis-nav-menu li:hover a,
.genesis-nav-menu li li a:hover,
.genesis-nav-menu .current-menu-item a {
color: #aaa;
}
….you can change the color and make other modifications from here.