Mugaga, ты такое хотел?
<!DOCTYPE html>
<html>
<head>
<style type="text/css">
</style>
<script type="text/javascript">
function asdf(o){
document.getElementById('test').style.backgroundColor=o.value
}
</script>
</head>
<body id='test'>
<form name="changeColor">
<select name="colors" onchange="asdf(this)">
<option value=""></option>
<option value="red">Red</option>
<option value="green">Green</option>
<option value="yellow">Yellow</option>
</select>
</form>
</body>
</body>
</html>