I've taken a look at sky background. Source images from NITE*.IMG are all around RGB 20,20,20 in flat dark-grey areas. Using a dropper tool to compare RGB values, I'm seeing the same exact same display output values in both classic and DFU. Flicking between screenshots from DOSBox and DFU on the same display shows the same sky colours for both versions of game.
I've also checked UI and the colours do render out very slightly differently (e.g. a value of 127 might come out 126 or 128). Not sure of cause there but it's practically imperceptible without using colour sampler. Even flicking back and forth between inventory images taken in classic and DFU do not produce any significant colour differences I can see with naked eye. Example below of straight screenshots to show that colours are being represented properly in UI in classic (left) vs DFU (right).
- inventory.jpg (299.5 KiB) Viewed 646 times
I don't see anything like the output in your photos on when comparing on same display. I understand this is probably down to differences in how an old display device is processing input (gamma response as you say) compounded with the difficulties in photographing the display to match what the eye sees.
I'm not sure I can do much more here. The comparisons shown are unique to your environment and photographs, and don't really represent DFU on a modern display. I think for sanity's sake, I can only tune using actual screenshots, not photographs of someone's display. That way madness lies.