Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js">
</script>
<script>
$(document).ready(function(){
  $("button").click(function(){
    $("li").toggleClass(function(n){
      return "listitem_" + n;
    });
  });
});
</script>
<style type="text/css">
.listitem_1, .listitem_3
{
color:red;
}
.listitem_0, .listitem_2
{
color:blue;
}
</style>
</head>
<body>

<h1>This is a heading</h1>
<ul>
<li>Peter</li>
<li>Lois</li>
<li>Chris</li>
<li>Stewie</li>
</ul>

<button>Add/remove classes to list items</button>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.