Sounds like a better solution than mine
You can use the photographic exposure lens shader, it has a saturation attribute.
From the documentation: "In photographic mode (a non-zero film speed) this is the factor used to convert between pixel values and candela per square meter. In arbitrary mode (a zero film speed), this factor is a multiplier applied when scaling rendered pixel values to screen pixels. This is analogous to the Gain parameter of the mia_exposure_simple shader."
Sorry for hijacking the thread but can someone explain what the top attribute "cm 2 factor" does in this lens shader?