I believe she was dressed in white in the Fatima apparitions but that in another such occasion she was dressed in a blue mantle, so that is undoubtedly part of the tradition as well, but I actually prefer the heavenly mantle explanation. St.Joseph is indeed a powerful intercessor for women with pregancy and childbirth problems. He saved MIL and both DH and his brother, who both have Joseph as part of their names. It is my personal opinion simply based on prayer experience of myself and others, that they still work closely together in heaven, and that he, like most good husbands, is smart enough to ask her help and direction when he is embarking on a good work.