View difference between Paste ID: LNX8FD5P and t7NQm9th

 <?php
 session_start();
 $color = (isset($_SESSION['color'])) ? $_SESSION['color'] : '#ffffff';
 ?>
 <body style="background-color: <?= $color; ?>">
-<button type="button" class="black" data-color="000000">black</button>
+<button type="button" class="color black" data-color="000000">black</button>
 <script src="js/jquery.js"></script>
 <script>
-	$('.class_name').on('click', function(){
+	$('.color').on('click', function(){
 		var value = $(this).data('color');
 		$.ajax({
 			url: "change_color.php",
 			type: "post",
 			data: {'color' : value},
 			success: function(data) {
 				$('body').css({'background-color' : '#' + value});
 			}
 		})
 	})
 </script>
 </body>
 ======================================================
 change_color.php
 <?php
 session_start();
 if (isset($_POST['color'])) {
 	$_SESSION['color'] = $_POST['color'];
 }