GammaThingy: Changes screen gamma on iOS, no jailbreak required

You'll need a Mac and Xcode to install (but you don't need to be a developer).

Instructions video: https://www.youtube.com/watch?v=D_01P4-ubdA

If you get weird blue and yellow circles,

=> turn OFF 'Reduce White Point' in Settings / General / Accessibility / Increase Contrast

Note: it works because you don't need to be a developer to load apps to your iPhone via Xcode anymore

Note2: it uses "private" APIs so it won't be accepted into the App Store (just like f.lux).