This extremely wide-brimmed style of hat originated
probably in tropical climates and served somewhat the same purpose
as a parasol - i.e., to keep the sun off the face. Not until
WW II was tanning considered fashionable for women, and a pale
or very fair complexion was considered the mark of a lady of
quality. But by the time of this photo, the practicality of
broad brimmed hats was of less concern than the striking appearance
it gave a lady. You couldn't help noticing a woman in such a
hat - rather a bold style. The ribbon around the crown might
be brightly colored also to add even more to the spectacle.
In any case the ribbon, even if just black, would add contrast.