Well, strange idea, and I have no idea how to make extension for that. But you can edit around line 567 in viewtopic.php
<div class="entry-content">
<?php echo implode("\n\t\t\t\t\t\t", $forum_page['message'])."\n" ?>
</div>
to
<div class="entry-content"><span class="group_color_<?php echo $cur_post['g_id']; ?>">
<?php echo implode("\n\t\t\t\t\t\t", $forum_page['message'])."\n" ?>
</span></div>
colors of post would be the same as You set in pun_colored_usergroups.
If any of my dropbox link fails with 404 error, change dl.dropbox.com/u/56038890/punbb/*.zip in address to 82283017.
Currently working on rPlus - responsive theme for developers to create their own themes basing on this one.