Strange tint in Prophoto B&W image.

[Redcrown]

Something strange happened when doing some B&W work. I took a landscape with dramatic sky from raw into Photoshop as 16bit Prophoto. I added a B&W adjustment layer, and lowered the Blue & Cyans to make the blue sky very dark. Then I noticed some blue/green tint in some areas. Actually, only in areas with RGB values between 26 and 31.

The tint is obvious on screen, but the info pallet shows no tint, all RGB values are equal. So I do a print screen and paste that into a new document. On that image the tint is still obvious, and the info pallet shows it. RGB values like 25-34-34. In all samples I took, Green and Blue are equal, but 5 to 10 points greater than Red. So, I'm starting to think my monitor is wacky.

But I do it all again in Adobe98 instead of Prophoto, and there is no problem. No tints anywhere, everything perfectly neutral. I go back to the Prophoto version and convert it to Adobe98. Volia, the tints disappear! But, the whole image gets darker in tone. The average goes from 97 to 84 on the B&W version, while the color layer looks unchanged. I convert the image back and forth between Prophoto and Adobe98 several times. The tint always appears in ProPhoto, never in Adobe 98.

Next I flattened the ProPhoto version, thinking something is happening in the conversion, but the tint stays in Prophoto and goes in Adobe98. So, I'm stumped. Anybody got an idea what's going on here?
 

[Stephen G]

following to see how this one pans out, but I do have a suggestion too: test GPU acceleration on vs. off, see if that makes a difference

[JRSmit]

What monitor, its settings, profiling?

[digitaldog]

We need a low rez version of each to view and examine.

[Redcrown]

Thanks Stephen G, turns out it is a GPU issue. With my GPU disabled in Photoshop, no color tint in B&W conversion. I didn't think to test that because I've never had a GPU issue before.

I'm on a Windows 10 (2004) Desktop, GPU is an Nvidia GeForce 1660, display is a BenQ SW270C. That's a 2560x1440 wide gamut monitor.

Still no idea why there is such a significant difference in brightness between ProPhoto and Adobe98 with a B&W adjustment layer, but that's not important. The lesson learned here is don't do B&W conversions in ProPhoto.

Here's is a sample of the tint:

[digitaldog]

If turning OFF GPU works, it's a GPU bug and you need to contact the manufacturer or find out if there's an updated driver for it.
Also see: https://helpx.adobe.com/photoshop/kb/acr-gpu-faq.html

This has nothing to do with RGB Working Spaces.
Try to recalibrate and build a new ICC display profile, the old one might be corrupted.
If you are using software/hardware for this task, be sure the software is set to build a matrix not LUT profile, Version 2 not Version 4 profile.