Xorg, but it has worked without issues on Fedora for over a year, and it also worked on Xubuntu before that :/
I know nvidia-settings messes up the colors if I open its GUI, but I haven’t found any other program which does so. I do use nvidia-settings from a script when power limiting + overclocking my GPU, but I have verified that it doesn’t mess with the colours, and I’ve also had it for almost a year so it’s not new.