As ~ultan said you can control with styles. Maybe some javascript too.
Code:
<script type="text/javascript">
function changeColors(sel) {
if (sel.selectedIndex > 0) {
sel.className = "green";
}
else {
sel.className = "red";
}
}
</script>
<style type="text/css">
.red option { color: #c00; }
.green option { color:#0c0; }
</style>
</head>
<body>
<form method="POST" action="do">
<p>
<select size="1" name="C1" class="red" onchange="changeColors(this);">
<option selected>.</option>
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
......
.....
...