Try jQuery:
<!DOCTYPE html>
<html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
$('#cars').change(function(){
$('#colors').prop('disabled', $(this).val() === '');
}).change();
});
</script>
</head>
<body>
<form action="">
<select name="cars" id="cars">
<option value="">Select one...</option>
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="fiat">Fiat</option>
<option value="audi">Audi</option>
</select>
<select name="colors" id="colors">
<option value="">Select one...</option>
<option value="white">White</option>
<option value="yellow">Yellow</option>
<option value="silver">Silver</option>
<option value="blue">Blue</option>
</select>
</form>
</body>
</html>