Improve color grading: Hue shift based on shadows/midtones/highlights

## What problem does this solve or what need does it fill?

I would like to hue shift to warmer color on midtones and cooler colors on shadows

## What solution would you like?

I would like more options in color grading (new in 0.14) to be able to do this

## What alternative(s) have you considered?

I guess i could write my own custom post processing shader ? :(

## Additional context

Start by locating Bevy's color-grading implementation introduced in 0.14 and the post-processing shader path mentioned in the issue. Establish how shadows, midtones, and highlights are represented, then define the expected behavior for warmer midtones and cooler shadows; no specific file, test, or entry point is named in the issue.

Written by the indexing model from the issue text.