Twilight and Lux both contain naive implementations using a semitransparent overlay. The result of this that blacks and dark colors end up being brightened instead of left alone, as a proper color-temperature change would do. This looks bad on all screens, but especially so on AMOLED displays that support a true black.

Cf.lumen[1] is, I believe, the only Android app that can actually change the color temperature of the screen like f.lux can, but it requires root as well as installation of a special "driver" in order to do so. My guess would be that the "complicated install process" referenced by the f.lux devs is probably similar.

[1] https://play.google.com/store/apps/details?id=eu.chainfire.l...