index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="./style.css">
</head>
<body>
    <div class="para" id="container">
        <p>hiiiii</p>
        <button onclick="changecolor('blue');">Blue</button>
        <button onclick="changecolor('green');">Green</button>
    </div>
    <div class="para1" id="container1">
        <p>hiiiii</p>
        <button onclick="changecolor('blue');">Blue</button>
        <button onclick="changecolor('green');">Green</button>
    </div>
    <div class="para2" id="container2">
        <p>hiiiii</p>
        <button onclick="changecolor('blue');">Blue</button>
        <button onclick="changecolor('green');">Green</button>
    </div>
    <script src="./script.js"></script>
</body>
</html>