Erreur dans programme

<!DOCTYPE html>
<html>
    <head>
        <title>Page Title</title>
        
        <style>
        
table {
   background-color: blue;
}

td {
   width: 50px;
   height: 50px;
   border: 5px solid blue;
   border-radius: 100%;
   background-color: white;
}

        </style>
        
    </head>
    <body>
        
        <table>
            <tr>
                <td onClick="testt(0)"></td>
                <td onClick="testt(1)"></td>
                <td onClick="testt(2)"></td>
            </tr>
            <tr>
                <td onClick="testt(3)"></td>
                <td onClick="testt(4)"></td>
                <td onClick="testt(5)"></td>
            </tr>
        </table>
        
    <script>
        
tour = -1;
function testt(i) {    
    td = document.getElementsByTagName("td")[i];
    if (td.style.backgroundColor=="white") {
        tour++;
        if (tour%2==0) {
           //si pair 
           td.style.backgroundColor = "red";
        } else {
           //si impair 
           td.style.backgroundColor = "yellow";
        }
    } else {
       alert("Pourquoi je m'affiche toujours ?");
    }
}
        
    </script>
        
    </body>
</html>