A Single Dress
I realize there are geographical differences of opinion, certainly if you live in New York feel free to use black, but in many areas you still risk hurting a bride's feelings if you wear black to her wedding. (I'm looking for a little back-up from you southern belles; most people around here look at me strangely when I suggest black is not appropriate wedding guest apparel.)
In any case, there are options. For a one-dress wardrobe, a seasonless dark, solid or print, with a summer sleeve (for me that is none) is ideal. Something simple and elegant, to be worn with pumps and a jacket in the winter and with sandals in the summer.
Having one go-everywhere dress on-hand doesn't mean I'm required to wear it to every event, it simply means I am not required to do emergency shopping.
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