I personally think that it is better if a PNM not wear black to pref. Many chapters will coordinate their members in black dresses so I think that it is nice if the PNM stand out a bit more and not blend in with the chapter. You probably wouldn't wear white to someone else's wedding so don't wear black to someone elses pref night.
I wouldn't wear black to prefs, because many chapters wear black. You want to stand out. PNMs at my school wore a variety of colors, I'm assuming other schools are the same.
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"Remember that apathy has no place in our Sorority." - Kelly Jo Karnes, Pi