While they may naturally be more likely to target dark objects, including hair, I think the scent of the hair can play a big part. Add to that the fact that they tend to get tangled in long hair and even the most peacefull bee can be incited to sting and release a scent calling other to her aid.
My wife has found that the perfume in some shampoos does attract bees far more than others. And while it doesn't appear that it makes the bees aggressive, they do get stuck in the hair which can lead to stings.
Of course if you are in an area where Africanized bees exist or may have crossed with European honeybees (as with the previous posters story), all bets are off (and hives do vary in temperament even without the influence of africanized genes, and can be set off given the right stimulus)
-Tim