Está basado en algo así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<script type="text/javascript">
onscroll=function(){
var sc=Math.max(document.documentElement.scrollTop,document.body.scrollTop);
if(sc>300)
document.getElementById('test').style.color='red';
else
document.getElementById('test').style.color='green';
document.getElementById('test').innerHTML=sc;
}
</script>
</head>
<body>
<div style="position:fixed" id="test"></div>
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</body>
</html>