Just to add a little onto the cookie method I suggest adding a class to the <body>
tag so that if in the future you want to do more you could do it without having to modify the PHP.
For example:
<?php
function dejavu() {
$class = '';
if($_COOKIE['beenHereBefore']) {
$class .= 'beenHereBefore';
}
else {
$class .= 'firstTimeHere';
setcookie("beenHereBefore", true);
}
return $class;
}
?>
<body class="<?php echo dejavu(); ?>">
One thing that you want to take into account though is that if a user clears their cookies then it will act as though they are visiting the site for the first time; so I suggest, if possible store it in their user profile if one exists.
So then in your CSS you can do the following:
ul li a {color: white;}
.firstTimeHere ul li a {color: blue;}