Here is my demo with jQuery
<!doctype html>
<html>
<head>
<style>
select{
color:#aaa;
}
option:not(first-child) {
color: #000;
}
</style>
<script type="text/javascript" src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script>
$(document).ready(function(){
$("select").change(function(){
if ($(this).val()=="") $(this).css({color: "#aaa"});
else $(this).css({color: "#000"});
});
});
</script>
<meta charset="utf-8">
</head>
<body>
<select>
<option disable hidden value="">CHOOSE</option>
<option>#1</option>
<option>#2</option>
<option>#3</option>
<option>#4</option>
</select>
</body>
</html>