[meue]

For your iPhone, you can use Color Filters under Accessibility -> Display & Text Size -> Color Filters. Set it to Color Tint and play around with the sliders to your liking (I personally set intensity to ~85%, hue ~8%, and compound it with Night Shift). I also use Reduce White Point (also located in the same place) to further reduce the screen brightness. With these two features, you can get your phone very similar to f.lux.

You can set these two features as active in Accessibility Shortcuts (bottom of the Accessibility menu), allowing them to be quickly toggled by triple clicking the power button. As an alternative gesture, you can also set Accessibility Shortcuts to activate via triple tap on the backside of the phone (Accessibility -> Touch -> Back Tap).