<ol id="commentlist">
<?php foreach ($comments as $comment) : ?>
	<?php // The extra stuff to get commenter's role
	$user_id = $comment->user_id;
	$role = ( isset( $roles[$user_id] ) ? $roles[$user_id] : '' );
	?>
	<li class="<?php echo $role; ?>">
	<p>By <?php comment_author_link() ?> - <?php comment_date() ?></p>
	<?php comment_text() ?>
	</li>
<?php endforeach; ?>
</ol>

Once done, open yout style.css file and paste the following:

#commentlist li { border:2px solid white; } /* not logged or subscriber */
#commentlist li.administrator { border:2px solid red } /* blog admin */
#commentlist li.editor { border:2px solid blue } /* editor */

http://www.wprecipes.com/wordpress-how-to-style-comments-of-every-roles

To do so, simply open your comments.php file and replace your comments loop by this one :

Wordpress comment style based on role

PHP