2 of 2 deleted 6 characters in body

You should do it in the grid template instead. You can add a new id to the order's row 'tr' . As an example, we'll highlight the orders placed in the last 24 hours.

Go to app/design/adminhtml/default/default/template/widget/grid.phtml. Starting in line 154:

<?php foreach ($this->getCollection() as $_index=>$_item): 
        $orderCreateAt = strtotime($_item->getCreatedAt());
        $last24hours = strtotime('24 hours ago'); ?>
        <tr id ="
        <?php if($orderCreateAt > $last24hours)
            { echo 'new-order'; }
        title="<?php echo $this->getRowUrl($_item) ?>"<?php if ($_class = $this->getRowClass($_item)):?> class="<?php echo $_class; ?>"<?php endif;?> >

Now defying the style in css should be straightforward. Go to skin/adminhtml/default/default/boxes.css and add the style you'd like to have

.grid table tr#new-order{
    background-color: #FF0000;