Untitled

<?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>
<script src="js/jquery.js"></script>
<script>
	$('.class_name').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'];
}