devo perderci le speranze?

Ho provato anche così, ma nulla:
codice:
<table>
<tr> <td id="cella1"> </td> ... ... ... </tr>
...
...
<tr> <td id="cella10"> </td> ... ... ... </tr>
</table>

...

<script type="text/javascript">

var cella1 = document.getElementById('cella1');
...
...
var cella10 = document.getElementById('cella10');


if (hours>17 && hours<20)
cella1.style.backgroundColor = "Yellow";

...

</script>