Hey,
ever considered the Ambience slider from the Google Snapseed app? It was part of my workflow and I am missing that a lot.
In my own terms, it works on chromatic and overall light contrast, enhancing the first while reducing the second. I was not able to find anything equivalent in AP. Note, it is a generic tool, backed by a mathematical formula just like contrast. The Snapseed app puts it in the level adjustments section (along with contrast, shadows, lights, saturation and so on).
I encourage you to download the app and take a look at how the live histogram changes while you add Ambience or remove Ambience.
ADDING - Chromatically, this seems to give some saturation to the picture (greens will be more green, reds will be more red), but at the same time, the intensity of each pixel slowly moves towards 127. This means that if you recursively add a lot of ambience, the master histogram starts to look like a triangle (with a growing peak right at the middle).
REMOVING - Chromatically, colors go toward each other, but at the same time, the intensity values will flatten on the whole histogram. This means that if you recursively remove a lot of ambience, the master histogram starts looking flat.
I am pretty sure this tools operates in the HSL space. But it is very hard to replicate with the HSL sliders in Photo. Please consider this! Also very useful if applied before converting a picture to BW, as it gives you easy control over the dynamic range of the BW image.
