Ok, I can answer this one: it baffled me for a very long time before the penny finally dropped. Your posts aren't actually coming out different from other people's. The boards are currently set up so that the LATEST POST on any thread will appear in a different colour from all the preceding posts. In point of fact, an annoyingly illegible pink. But only the first time that one looks at the thread. The next time you visit the thread, you'll find the post has reverted to the default purple colour. Try it and see. It's a funny feature of the system, which I suppose is meant to highlight the latest posts, though in practice it just makes them hard for you to read!
But as 24601 and BHG say, you can get around this if it bugs you by manually specifying the font colour of your post via BB code -- this overrides the software's determination to cast your latest remarks in the rosiest possible light.
If you edit a post, it will have the same effect of temporarily making it appear pink to you!
Another oddity of the system...this doesn't seem to be true for all threads. Or for all users. We're still trying to figure out exactly when and why this happens. Are you using a Mac?
“When did you ever promise to kill yourself falling out of Charlie’s tree?”