I don't think my schoool had any rules about what you could or couldn't wear with your (plain black undergrad) robe. The only stoles or cords we wore, though, were from honor societies - no one wore fraternity or sorority stoles.
Our undergrads, too, all wore the same color robes; only the masters and PhD candidates wore different colors. But I loved seeing the profs in their robes! Tartan ones, bright pink ones ... we tried to guess where they were all from.
You can see my sister Mindy here decked out in all her cords:
http://www.geocities.com/axdtm/calendar.html.
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