Highlight Table Row On Selection?

I have a php script that lists work shifts in a table I have modified it with a drop down box that jumps to a specific date on selection, what I now want to do is highlight the selected row when its chosen from the drop down box how can I go about doing this? Current javascript present on the page (not written by me).

Code:
<script type="text/javascript">
window.onload=function() {
// hide list
var objBM=document.getElementById('bookmarks');
objBM.style.display='none';
// create dropdown
var container=document.createElement('div');
var sel=document.createElement('select');
container.appendChild(sel);
objBM.parentNode.insertBefore(container, objBM.nextSibling);
// onchange
sel.onchange=function() {document.location=this.value;};
// fill options
var aBM=objBM.getElementsByTagName('a');
sel.options[0]=new Option('select an option', '');
for(var i=0; i<aBM.length; i++) {
sel.options[i+1]=new Option(aBM[i].firstChild.data, aBM[i].href);
}};
</script>

And each row is either:
PHP Code:
echo "<tr id=".$i." style="font-size: $fontsize;" bgcolor=$oddcolor>";
or
PHP Code:
echo "<tr id=".$i." style="font-size: $fontsize;" bgcolor=$evencolor>";
So each row is given an ID number based on its record number, so it should be as easy as just changing bgcolor of a given id no. But I only want one row highlighted at a time, so if a user selects a new date, the new one will be highlighted only.