Film Simulation: 2 - Colour (Theory)

The previous post showed how to produce simple B&W conversions through the use of the channel mixer with fix channel proportion values, to mimic the behaviour of specific traditional, chemical B&W films.
Now, with colour, we have a different topic here; chemicals were laid in fact in three different layers, and developed "independently" in a single process. Any small variation in some parameter (products, chemicals concentration, temperature...) would lead to slight mismatch in colour output per channel.
Mastering colour film development was complex, far more than black and white development. Getting each layer (exposed to different colours from the beginning, thus with different light intensities "per channel") to develop up to the right point could take years to learn...
But even if we did develop the film properly, we would find that each film brand and type would need (in fact, similarly to B&W film) a specific process. This was caused also as a "conscious" manufacturer decision. A same "family" of films shared some common behaviour parameters in terms of saturation, stress in specific colour or contrast enhncement.

First approach...
So each colour channel reacts in a different way to light in the original image, then chemical development itself was not linear (faster at the beginning, slower at the end...). So depending on the subject and its colours, the chemicals in the film and the development process we would get different results. How to do this?
Basically, we should make a first curves adjustment per channel, then an overall adjustement (RGB common value) to get the desired contrast level. In GIMP, we might want to do this with a duplicated layer, then control the fusion mode to the background image to adjust the intensity of the edition...

Next post: Film Simulation: 3 - Colour (Cross Process)