Lightroom Classic changing colors on import?

[michaelm79214895]

I've created a set of files inPhotoshop 25.3.1 with color patches in pure red, pure green, and pure blue.  Four patches each, at 25% / 50% / 75% and 100%.  In Photoshop they are exactly as described, pure red / pure green / pure blue.  They were saved as TIFF (I also tried PSD and JPEG) in  AdobeRGB.  [Attached to this post is the JPEG version of the blue file]

I've then imported these files into Lightroom Classic, and the color patches no longer appear pure but a mix of red / green / blue as illustrated by the histogram data [attached].

Opening this file again from Lightroom Classic into Photoshop ("Edit in...") shows the file is untouched, i.e. the color patches remain pure (blue, in the case of the attached file).

However, exporting this file from Lightroom Classic as e.g. a TIFF ( I have also tried this with PSD and JPG) using Adobe RGB creates a new file in which the color patches are no longer pure, but a mix of red / green / blue.  No editing was done to the file, no preset applied, no camera profile, no white balance changes, nothing. [Attached to this post is the JPEG export].

I have also replicated this identical process with Photoshop and Lightroom on iPad, with identical results, i.e. a shift in colors following import into Lightroom.

Is this a bug?

[JohanElzenga]

I have downloaded the first two attachments, opened them as layers in Photoshop and set the blending mode to 'Difference'. The result is pure black, which suggests that there is nothing wrong with these files. They are identical. Perhaps this is a problem with your monitor profile.

[michaelm79214895]

Good point, and important to check / verify.  However I assume the monitor profile would not affect the histogram that Lightroom / Photoshop calculate?

[JohanElzenga]

Good point, and important to check / verify.  However I assume the monitor profile would not affect the histogram that Lightroom / Photoshop calculate?

By @michaelm79214895

The histogram differences are explained by John. Contrary to what you think, conversion from AdobeRGB to ProPhotoRGB does change the RGB values. Think about it: if you create a patch that is maximum blue in AdobeRGB, then that would be RGB 0,0,255. That color is not maximum blue in ProPhotoRGB however. Maximum blue in ProPhotoRGB is a much more saturated blue. So if you convert AdobeRGB 0,0,255 to ProPhotoRGB, then the result will be different values. It could become something like 0,0,212 for example, but it is also possible that the R and G values are no longer zero either, because these color spaces do not only differ in size, their shapes are also not 100% identical. You can try this yourself in Photoshop. Create an empty AdobeRGB document, fill it with RGB 0,0,255 and then convert it to ProPhotoRGB. Then check what the new color values are (I would have done that, but I am typing this on my iPad).

[JohanElzenga]

Update: I just did that test and I think you will be very surprised indeed. RGB 0,0,255 in AdobeRGB becomes RGB 88,36,241 when converted to ProPhotoRGB!...

[michaelm79214895]

I applaud you JohanElzenga for persisting with this -- thank you.  My assumption was that a color 0/0/255 in AdobeRGB would map to something 0/0/x in Prophoto RGB. Why should moving to a larger space cause elements of red and green?

If however,, as you have shown with your test, Prophoto RGB is not just larger than AdobeRGB, but also has a different shape, then I would indeed expect the combination of R/G/B to change.

In other words: thank you!

PS link to an article that includes an illustration of the different color spaces https://learn.zoner.com/srgb-prophoto-rgb-and-more-do-you-know-your-color-spaces/

[John R Ellis]

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Photoshop's histogram of the image "BLUE (PS Adobe RGB 8bit).jpg" shows the pixels aren't purely the blue channel -- there's a significant amount of red and green channel as well at the darkest levels:

 

You don't say precisely how you created the image, but likely you created it in a different working color space and then converted it to Adobe RGB when you saved it.  An RGB value of (0, 0, n) in one color space isn't guaranteed to have 0 values for R and G when converted to another color space.

LR's working color space uses the primaries from Prophoto RGB. When I convert the image in PS to Prophoto RGB, the PS histogram looks similar to what you see for the original image in LR:

If you create a blank test image in PS with the color space set to Prophoto RGB, then set the pixels to RGB values of (0, 0, n), then save the image with color profile Prophoto RGB, then open it in LR, you should see the same histograms, with just blue-channel pixels.

[michaelm79214895]

Thanks for the detailed response.  Like you, I first suspected a change in color profile somewhere along the chain, and I will certainly re-trace my steps once again as soon as I am back in the office.  In the meantime, a provisional answer:

I had created the file in PS in Adobe RGB color space and saved it with this profile attached as a TIFF.  I would have expected that a conversion from Adobe RGB to Prophoto RGB (which is significantly larger) should not result in any changes?

What I will do once back in the office later today / tomorrow morning (UK time) is to re-do this whole chain of steps.

Update to follow