How can I correct for GND color cast between filtered and unfiltered areas, in post?

Question

From How to Correct GND Filter Effects in Photoshop for Consistent Exposure and Color? in the /r/photography subreddit:

I have captured an image using a GND filter, which has resulted in noticeable differences in white balance, color temperature, and exposure between Area A (unaffected by the filter) and Area B (impacted by the filter). There are obvious disparities that need addressing. Could you guide me through the process [...] to harmonize Area B with Area A, ensuring that the cloud details remain intact and that the cyan tint and fog-like dark shadows are eliminated?

[I want] to achieve a seamless and natural transition between these two areas in the image.

The question was originally asked with regards to Photoshop, but any photo editing tool that can achieve the goal applies.

The referenced image in the subreddit is the example photo in Wikipedia's graduated neutral density filter article:

^{GND Demo, by Wikimedia Commons user BenFrantzDale~commonswiki. CC BY-SA 3.0}

Answer 1

Just me, but you're approaching the task the wrong way with the wrong tool.

The simplest and easiest way to address this issue is to a) get yourself a GND filter that's actually neutral if it's adding a color cast, or b) ditch the GND filter altogether and reshoot.

And if you still need to post-process to counteract a GND filter, the easiest way would be to shoot RAW and use the graduated filter in Adobe Camera RAW, not Photoshop.

Answer 2

Re-doing colour balance in Photoshop is kind of bad idea since there are some colour modifications (LUTs) applied when developing. In a perfect world I'd love to re-do the raw data in Photoshop or GIMP and then use it in place of original data.

But typically multiplying image data with some colour will give you pretty good white balance method when it's not strong when you exported the image with neutral tonal curve (and preferrably a linear DCP profile). The problem will be that you will need to choose the correct gradient middle points.

Besides that I do not know if Photoshop allows to do multiplication in linear gamma (like GIMP does).

If you absolutely want to nail it you can photograph a white sheet and use it to create a correcting gradient after you develop the file with same settings.

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noticeable differences in ... exposure

Isn't that the whole point of GND? But if you prefer you can then use another grayscale gradient in multiplication mode to decrease brightness of non-darkened part.