index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Color hex</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div class="container">
        <span class="color"></span>
        <button class="generateBtn">GENERATE</button>
    </div>
    <script>
        const btn = document.querySelector('.generateBtn');
        const color = document.querySelector('.color');
        const setBg = () => {
        const randomColor = Math.floor(Math.random()*16777215).toString(16);
        document.body.style.backgroundColor = "#" + randomColor;
        color.innerHTML = "#" + randomColor;
        }

        btn.addEventListener("click", setBg);
        setBg();
    </script>
</body>
</html>