Greenified for Android is based off an old web service I use to run that would turn webpages to black and white on the fly. For your computer/device screen to show a bright pixel, such as White, it takes significantly more power than a darker pixel. On mobile devices, the biggest battery killer is the display. By applying a darker filter over your device screen, your power is greatly saved while the screen is on, as it requires less power to display everything.
Greenified allows you to apply a filter over your screen to reduce the brightness even further than the Android operating system allows. You are able to pre configure settings, including screen intensity and a brightness setting. When running, all settings are applied instantly, and restored to the previous state on stopping.
Please note, on some devices this may (as will any screen filter app) prevent the Install or Uninstall button of the Package Installer from working. You may need to disable the app before installing/uninstalling apps. A future release will detect when the package installer is active, and disable itself.