Untitled

<?php
session_start();
$color = (isset($_SESSION['color'])) ? '#' . $_SESSION['color'] : '#ffffff';
?>
<body style="background-color: <?= $color; ?>">
<button type="button" class="color black" data-color="000000">black</button>
<script src="js/jquery.js"></script>
<script>
    $('.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'];
}