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Perhaps it is a green varietyPerhaps it is a green variety of pumpkin! In Australia lots (most) of the commonly grown varieties of pumpkin are green, grey or speckled. If the pumpkin you started with was a hybrid perhaps it has reverted to type? If this is the case then you should expose it to sunshine after it has been cut if you want to store it but just having one I would just eat it straight away.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?