My envelopes are different colours (grey, blue and pink) as well as being open or closed, but I have no clue what that means; is there a FAQ or similar somewhere explaining what the various icons throughout this forum actually mean?
You often see some kind of legend on different forums, but it looks like Vbulletin decided against that idea. My take on it is that blue is used for threads with unread posts and that it turns grey (and the envelope closes) when you've read them all. Unfortunately, on some monitors there's not much difference between the blue and grey, so the envelope being open or closed is sometimes clearer than the difference in colour.
The exception would appear to be the "hot topics" which are red/pink, a hot topic being one with more than a certain number of posts. It looks like these stay red even when you've read all posts, so then you really do only have the envelope image to go on.
This is all just based on my own observations, though - I could well be reading them all wrong too ;-)