I've heard that it can take 6-8 years for the larger conures to attain their full adult coloration. Since my first two conures were easily in their pre-teens (if not older) when I got them, their coloration has not changed.
My third conure was about 3 1/2 when I got her and she actually looks more green now than when I first got her! So I suspect that her coloration *will* change as she gets older! (and she ought to be about 4 now)
Do all baby suns start out green? No.... but most do.
Pied sun conures may be born pretty yellow, and the more they are bred to be yellow, the more the chicks are born yellow. You can see in the following thread.