technically, if you want to where a hood, you have to wait until after you've received your diploma. The main graduation here doesn't use hoods (because there's usually about 6000 people in a class) but at the graduations for each academic college, or for department ceremonies, they will "hood" the graduates.
The colors and shapes of robes hoods are standardized across all US universities and colleges- if you know what you're doing, you can "read" a person's regalia and know where they graduated from, with what degree and major, and their honors status. It's all here:
http://www.acenet.edu/faq/costume_code.html