I have seen so many posts regarding the same but all are outdated or not working anymore.
I am using iOS-Runtime-Headers and everything is just nicely setup. But it said that I can't find out the method in which I can set iPhone's screen display color. This should be possible with private APIs as long as, we've NightMode from iOS 9.3.1.
And yes, I am aware that if I will be using any private APIs, Apple will surely reject it.
Have you check out the GammaThingy project out?
It works a bit similar to f.lux and NightShift and maybe the code of the GammaController there can help you achieve what you want