[Sweet]

Предлагаю следующую реализацию (все в одной функции):

<script type="text/javascript">
var superF = (function(){
  var storage;
  return function(){
    var event = arguments[0] || window.event
    switch(event.type){
      case 'click':
      if (storage && storage != this) storage.style.background = "white";  
      this.style.background = "green";  
      storage = this;
      break;
      case 'mouseout':
      if (storage != this) this.style.background = "white";
      break;
      case 'mouseover':
      if (storage != this) this.style.background = "#39F";
      break;
    };
  };
}());

var n = 3;
document.write("<table border='1' style='border-collapse:collapse; width:400px; height:400px;'>");
for (var tr = 1; tr <= n; tr++) {
    document.write("<tr>");
    for (var td = 1; td <= n; td++) {
    document.write("<td align='center' valign='middle' onclick='superF.call(this)' onmouseout='superF.call(this)' onmouseover='superF.call(this)' id='tdcolor'>A</td>")};
    document.write("</tr>")
}
document.write("</table>");
</script>

UPD: подправил передачу события