A green papaya is just papaya that isn't ripe yet. It's deep green in color and very firm to the touch. Unripe papaya has an off-white flesh and a mix of white and gray seeds inside. This version of doce de papaia calls for unripe, green papaya but there are versions of doce that use ripe papaya. Neither version is more 'correct' or more 'authentic'. It's simply a matter of preference. I personally prefer to make papaya jam with green papayas, because the texture of the green papaya holds up better during cooking.