It's so funny how things vary from school to school. When I lived in, we didn't have to have matching stuff in our rooms, but over the years, our chapter has evolved to the point to where the roomies like to have a matching room, and they plan it out over the summer.
As for alumnae attire for alums assisting with collegiate recruitment-- when I assisted with recruitment if I was in the front serving water, etc., I was prior given a list of what the girls were wearing, and asked to match (ie: if the girls were wearing baseball tees and denim capris, I could wear a baseball tee or 3/4 length shirt and denim bottoms of my choice; if the girls were wearing pref attire, I was asked to dress in pin attire, preferably all black.). If I was doing behind the scenes work, there was no dress code, usually something comfy.
The safest bet if you are a PNM going through recruitment: look your personal best and if you consider an outfit questionable-- either too casual or too "revealing," leave it in your closet. You're not going to offend anyone by wearing those clothes, but you won't be dressed to suit the occasion, either. This isn't a beauty pageant or a competition, but looking your personal best makes you feel better and when you feel your best, it shows. As some other sisters at GC have recommended-- a fun coversation piece like a unique pendant or bracelet helps as a conversation point, sometimes. Of course, remember that it's not what you wear, but who you are that makes you the most memorable
So make a laundry list
of all the wonderful and special things about you and make sure that the sororities know about those things!