Decided to look into why color grading doesn't retain when FSR is enabled, even if "Disable for Screenshots" is checked in the CSP settings.
Long and short of it is - the button doesn't do anything, at all.
However, you can change this in the .ini itself!
Navigate to
assettocorsa\extension\config, then find and open
graphic_adjustments.ini
Find the FSR section in this file and change
"DISABLE_FOR_SCREENSHOTS" 1 to 0.
That's it! It works with the game running too so you can check and validate (confirmed working on CSP 0.2.5-p1).
Below you can see that the screenshot was taken with the Fuji Pro 400H color grading and its retained through the screenshot.
Enjoy!