When it comes to motorcycles and style, nothing quite does the trick in the helmet category as a Leather Motorcycle Helmet. Typically there are two distinct types of helmets here (no, not black and brown);
Motorcycle Helmet that is covered with black or brown leather or, (usually DOT rated)
A vintage motorcycle helmet that is made out 100% out of black/brown leather. (not DOT rated)
Helmets that are made 100% out of leather typically do not carry any type of safety certification such as a DOT cert. They are often referred to as motorcycle hats or caps.
There would not be very much benefit to wearing a leather cap such as these related to safety. They would not provide very much protection in the event of a crash. Although a low speed crash or some other light impact they might prevent a few bumps, skid marks, or even road rash. Better than not wearing a helmet at all! Just don’t try and pass one off for a real helmet if your state requires one.
It’s really more for that look, in a vintage kind of way.
Here are a few pics of some old school biker ladies wearing their leather motorcycle helmets, and a few other vintage photos from that time period.