The first time I was in Minab (south of Iran), I was wondering why in that unbearable extreme hot weather the women put on Borke (Mask) and then two travellers asked me about that; I said probably it's because they are very religious. But, I was wrong, at least to a degree. Some believe that it's a tradition dates back to the time when Portuguese were in Persian gulf, but, more probably it's a tradition dates back to ancient Iran (before Islam); neither is for sure.
That Borke is for protection from hot weather, probably their skin would burn (and that is important for women). And from the colours of Borke, you can find out a lot of things; purple Borke means the girl is unmarried, red ones mean married or just married, orange ones mean they are married and have children, and dark ones mean aged and maybe widow. I am not very sure about colours like others.
All over the south of Iran, you can find some masked women, but, I would say the Bazaar of Minab is the best place for seeing and finding out more about Borke.