//////////////// In css file ///////////
.box {
    background-color:#ffffff;
    width:50px;
    height:50px;
}

//////////////// In head /////////////
  <script type="text/javascript">
  function playerColor(element){
    element.style.backgroundColor=#ff0000;
    return;
  }
  </script>

////////////// In body ///////////////
<td class='box' onclick="playerColor(this);"></td>