I don't think there is a problem. When a user visits a forum and there have been new posts since their last visit the purple icons should show up as new posts. They should remain that way until they leave or "Mark all topics as read" on the active topics page.
The purple icons could also be showing because the images are being cached either by the users browser or their ISP's proxy server. Holding down CTRL-Refresh or CTRL-Refresh may force the browser to get the proper images.
Did you get the email I sent you yesterday or the day before?