If you look at a map of temperate regions where tobacco was successfully and extensively grown commercially during the mid 19th century, that coincides fairly well with growing areas where the growing season is long enough for outdoor germination, and where color-curing is really easy. Although I'm in Virginia, I'm up above the Blue Ridge. So it wasn't a natural location for tobacco production. By pure luck of my micro-environment, the compass orientation of both prevailing winds and my old farm shed, as well as the accident of that old shed having ideal ventilation, and heat reflection from the exterior wall of the house--color-curing is easy for me.
I agree with the consensus that the yellower the leaf in the field, the easier the color-curing. (That's usually the way to go for cigarette, pipe and smokeless tobacco. For cigars, there is a delicate dance between ripening and leaf damage.) A few years ago, I fiddled with a pile of fresh banana peels inside a trash can in which I hung Oriental tobacco. In comparison to control plants, I was not impressed with a significant acceleration of ripening. Maybe the peels weren't releasing enough ethylene, maybe the temp wasn't ideal, or maybe the tobacco was just being contrary.
Bob