I have a simple solution for future reference (probably mine as well next time I have to google this!)
My settings and issues match exactly with the original post - Rec 709 2.4 project. When viewed in Preview, exported stills have a gamma shift making them appear slightly washed out. This is the same whether exporting stills from the Color page or exporting a TIFF sequence.
The issue appears to be that MacOS/Preview can't read the colour profile information that Resolve tags stills with. This can be confirmed by viewing the file info (CMD+I) - a comparison of this info before and after the fix is below.
- Screenshot 2023-03-31 at 11.03.47.jpg (27.62 KiB) Viewed 1907 times
I discovered that Photoshop reads the colour profile settings correctly, and I also know that when you save a file out of Photoshop that Preview/MacOS will see that colour profile information. So the solution is to either force Photoshop to tag the file as Rec.709 / Gamma 2.4 in a way that MacOS will understand, or actually bake in a conversion from 709/2.4 to sRGB. Both will work, but I opted for the latter as a way to hopefully avoid further issues if those stills ever get opened up on a computer that can't read the 709/2.4 tag properly.
So that's the theory. To apply the fix, open the still in Photoshop and go to
Edit > Convert to ProfileA window will open that should tell you that the Source Space is Rec.709 Gamma 2.4.
Either change the Destination Space to Rec.709 Gamma 2.4 or change it to sRGB - up to you as per the explanation above.
Save the file.
Mac OS/Preview will now correctly interpret the colour profile.
I haven't attempted to set this up as a batch action but presumably it's possible for anyone dealing with lots of stills.