Could have been either fall colors made green or summer colors made into autumn.
An absolutely beautiful shot and the transition couldn't have been done better.
Been trying to find behind the scenes answers on it but no luck so far. Sure a change from the old days even as early back as Dances with Wolves where they had to physically paint autumn into the battle at the beginning of the film.
Well if you can't simply key/change the colors around, then another way of doing this is via a matte painting. This is easiest if the camera does not move (otherwise the trees need to be 3-D). Someone in Photoshop (or similar program) just creates/paints both versions of the trees.
If you want falling leaves, those elements might be 3-D leaves added in. On the leaves, you would change the texture colors around.