these forums colors are based on templates that can be changed to a great extent. I suspect that some changes where made to the original forum template early on. after the last round of updates/upgrades/reinitialization of the forum it seems maybe the original forum template settings where reinstated and those are now being used. the old settings are still retained for users posts because the the forum either needs a cache clear or it is just a feature or the forum to retain old settings even if new ones are set.

basically saying that the forum was changes to say new members are this color but after the updates users started to get a different color.

ive setup a handful of online forums before so thats my best guess. if anybody wants to make an admin for a little bit I can maybe take a look at fixing some stuff. but really "it ain't broke" so....