Bumping this because it's the time of the year.....and summer sale is still on.
Please read the opening post for some idea on what people tend to wear for matriculation. The source is a few years old, but I assume there hasn't been too big a change.
Also, bear in mind some of the photos were taken a few hours before the actual matriculation ceremony (all to do with colleges/photographers scheduling things...), so it's possible they were wearing something a bit different (most likely more formal) for the ceremony itself.
Also, possible different dress code for matriculation dinner, which is usually the previous evening.
You will get all the necessary info on your college's dress code very soon in Welcome Pack you get from your own college. So please refer to it, too, when you get it.
Also, your college's JCR and/or college parent/s will be get in touch with you before too long via email. So if there's anything else you want to know in detail, you can ask them. That's they're there for, for next 3/4 years.