In your skin1/customer/categories.tpl
find
Code:
{foreach from=$categories item=c}
<font class="CategoriesList"><a href="home.php?cat={$c.categoryid}" class="VertMenuItems">{$c.category}</a></font><br />
{/foreach}
and replace with
Code:
{foreach from=$categories item=c}
<font class="CategoriesList"><a href="home.php?cat={$c.categoryid}" class="VertMenuItems" {if $c.categoryid eq 4}style="color:#ff6ae0"{/if}>{$c.category}</a></font><br />
{/foreach}
Insert desired category number and color istead of 4 and #ff6ae0