I've heard this before, don't know how true it is though. From my side of recruitment, in my chapter, we really did not care if a girl wore black (even though we all did during pref), but other chapters/campuses may consider girls in black "lost in the crowd". If her sister is on the same campus as her, I would heed her advice.
Wearing a color can help you stand out, BUT if she's going to be hideously uncomfortable in color she'll stand out in the wrong way.
See if she can find something in maybe a deep plum or navy (i.e. does a lot of the nice things black does for you without actually being black). She might need to get out of the junior department for that.
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It is all 33girl's fault. ~DrPhil