Before a recent patch change, the Colossus used to go obsolete upon Navigation:
Goes obsolete once any player researches the Navigation technology.
Pretty clear wording.
The Great Wall, however, is a little more ambiguous:
Upon discovering Dynamite, The Great Wall becomes obsolete.
The question is, whose discovery of Dynamite? The owner of the wall? The enemy trying to walk inside the wall's owner's territory? Any player, like the Colossus?