As someone who attended Catholic School, I assure you that the reason that nuns wear black is because it enables them to creep up on you from the shadows and…[BLINK]POUNCE!![/BLINK] 
You should tell the truth, expose the lies and live in the moment."-Bill Hicks
“You should tell the lies, live the truth and expose yourself.” - Bill Clinton
my wife insists that they’re mourning the loss of a possible sex life… 
I don’t suffer from insanity…
I enjoy every minute of it!
It’s slimming.
Uke
It’s easy to accessorize, goes with everything and looks STUNNING with ivory rosaries, dahlings.
—Sister Schiaparelli of Paris
Just like Friars and monks, nuns wear habits according to their order. Clarissans wear grey (while their Franciscan male counterparts wear brown) as do I suppose Benedicine nuns (verification?). Brigitines (Augustinian nuns) wear black or gray, and perhaps Dominican nuns also wear black (per Catherine of Siena). Some other order wears white, although I’m not sure who (associated perhaps with the Capuchins?). Perhaps the notion that all nuns wear black stems from the fact that the most that we associate with (the ones at your Catholic grade school) are from the teaching/preaching orders (i.e. Jesuit, Dominican O.P., etc) and thus happen to wear black.