That was a question I was asking some time ago as well (in a rant somewhere about illogical redecos just to give retailers "new" stock), because last year we got two green Optimus toys in Beast Hunters...
Maybe a homage to the BAPE green Convoy perhaps? I don't recall that colour being used before for a Hasbro Optimus toy (or in fiction).
Usually it is white or black as a redeco, so this green colouring is an unusual choice, and having it twice in the one year makes it even stranger.
I've added it to my BotCon questions list, to see what the Hasbro designers/colourists have to say.