So, when you switched OS, you got the same result as swapping cables? Or did you switch to DP on Nobara as well? This part confused me. Anyway, I don't know if windows recognises the display differently when you use HDMI and DP (other than the different technology of course) but something similar happened to me when I switched from hdmi to dp a couple of years ago, I had completely different colours, and I fixed it by changing the windows color properties, display calibration and some tweaks with the Nvidia control panel. I think it's because you are used to that image now so try to match it as much as you can with these options. If nothing works try different dp cables, sometimes you get faulty ones.