<?php
$all_fieds = get_post_custom( $post-> $id );
for ($i=1; $i<=5; $i++) {
if (array_key_exists('mod'.$i, $all_fieds)) :
?>
<li>
<div class="color_item_container">
<img src="<?php echo get_field("mod".$i); ?>" height="50" width="30" alt="alt">
</div>
</li>
<?php endif;
} ?>