[Will Hayes]

You could try

{$cat}

This variable contains the parent category id of the current category or product page you might be viewing.

For example, you could loop through your standard category array
$categories

and perform a check for a match like this:

{foreach from=$categories item=c}
<li class="color-{$c.order_by}"><a {if $cat == $c.categoryid}class="active"{/if} href="home.php?cat={$c.categoryid}">{$c.category}</a></li>
{/foreach}

Notice the if statement within the a tag.

Hope that helps you somewhat.

regards
W.