devi fare così:

<table>
<tr>
<td onMouseOver="this.style.background=('red')" onMouseOut="this.style.background=('')">Primo valore</td>
<td onMouseOver="this.style.background=('blue')" onMouseOut="this.style.background=('')">Secondo valore</td>
<td onMouseOver="this.style.background=('yellow')" onMouseOut="this.style.background=('')">Terzo valore</td>
</tr>
</table>