Indeed i haven't been clear enough, i made a sketch of how my world is designed to help you picture this.
To answer your questions, all the layers (both backgrounds, ground, foreground, sky) have to scroll vertically and horizontally with a different parallax ratio. Actually, backgrounds aren't animated and they won't "change" during the gameplay. The transition between colors has to be the smoothest to indicate the players where they are going at. My camera is completly free, following the player, and there is no cinematic yet.
Also, I'd like to make a day/night cycle but i already have all my ideas, i'll be using a vignette effect, lowering luminosity and contrast.
Thank you for giving interest to my request !