Mine was a sundress but our recruitment was low on frills and that was the only day that that level of dressiness was expected.
This is the only day that any sort of formalness is expected at my school as well. I think I'll try to contact someone in the Greek office and ask them.
FWIW, don't wear plain black or plain white without that splash of color you mentioned. Many of the GLOs wear all black or all white for Pref, and you want to look as if you would fit in nicely, but not presumptuous. Good luck to you!
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