As I see it though, after choosing the film all we have control of are the densities involved.
My thought is simply that for any one given film there is only one color hue available on each layer and that when the three layers are "balanced and processed normally", the sum of the layers will always reach full design saturation.
I guess one way of saying this is that, even a perfect gray caught on color film isn't a monochrome or desaturated color, it's a fully saturated real world color created by a "balance" of density between all 3 layers.
We can manipulate contrast through composition, lighting, choosing the exposure relationships between our subjects and their settings, with polarizing filters....
We can manipulate the exposure relationships (balance) between the layers with colored filters to skew the densities one way or another to manipulate color balance. This is generally global though, with all the hues shifting say warmer or cooler, it may look better/provide better color contrast/be just what we want, but the saturation hasn't changed and our perfect gray subject from the scene is no longer gray.