If I remember correct it's a style setting. There is a style for topics with new posts.
I have not been able to find this option in the new styles or editor. Just using newfolder/nonewfolder icon doesn't work either. I've put in a bugs report on that one. Anyway, if in fact an option does exist via the style editor - where is it ? I've located option to color subject line of new topics but not new replies.
Thanks for help
hikelite aka packlite