bgcolors.html

<!DOCTYPE html>
<html>

<head>
    <title>
        Script
    </title>
    <style>
        body {
            margin: auto 0;
            height: 100vh;
            display: flex;
            align-items: center;
            justify-content: center;
        }

        button.in {
            outline: none;
            border: none;
            width: 250px;
            height: 100px;
        }
    </style>
</head>

<body>
    <button type="button" class="in">Change color</button>
    <script>
        var colors = ['blue', 'green', 'tomato', 'yellow', 'purple'],
            i = 1;
        var elementToChange = document.body;
        var x = document.querySelector(".in");
        x.addEventListener('click', function () {
            elementToChange.style.backgroundColor = colors[i++];
            if (i === colors.length) {
                i = 0;
            }
        });
    </script>
</body>

</html>