Per the small text at the bottom of the poster I would say they are based on shipment volumes -so how much is available. The lighter colour probably indicates variation depending on how good the growing season is

I guess the second color is the margin of error or wiggle room on the prediction.

There's no y axis labeled or marked, the visual is vertically spreading each curve out so it's not all one big color jumble. Not sure what the second color means exactly either.