If you take a look at a pm list (e.g. the pm inbox) on a clean phpbb3 board (including phpbb.com) using IE6 and prosilver, you'll see that every pm that is not supposed to have colored left and right borders (i.e., every pm that is not from a friend or a foe, has not been replied to and has not been marked important) is marked with almost black sidebars even though the actual html code for the row has no left or right border color.

AdamR in a separate topic has already traced this to the following code in colours.css:

ul.topiclist li {

	color: #4C5D77;

}