così?

codice:
<html>
<head>
<script language="javascript" type="text/javascript">
function cambiaColore(cell) {
  document.getElementById(cell.id).style.backgroundColor = '#0000FF';
}

function ripristinaColore(cell) {
  document.getElementById(cell.id).style.backgroundColor = '#EEEEEE';
}
</script>
<style type="text/css">
<!--
.default {
	background-color: #EEEEEE;
	width: 300px;
}
-->
</style>
</head>
<body>
<table width="600" border="1" cellspacing="0" cellpadding="0">
  <tr>
    <td id="cell1" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
    <td id="cell2" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
  </tr>
  <tr>
    <td id="cell3" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
    <td id="cell4" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
  </tr>
  <tr>
    <td id="cell5" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
    <td id="cell6" onMouseOver="cambiaColore(this);" onMouseOut="ripristinaColore(this);" class="default"></td>
  </tr>
</table>
</body>
</html>