Be aware that some pumpkins are green! It depends on the variety. I have found that the really orange ones have already turned a pale yellow shade once they start to grow. If they are big and green, I would say they will stay green! They tell you themselves when to pick them - when their stem is hard and dry and it will just detach from the vine with a small movement. I just picked one Turk's Turban, but they are precocious and that was the first fruit. Most of my pumpkins are not ready yet and won't be for another few weeks.