There is a free and premium version available. The free version works just fine for me, I’m not sure what the benefit of upgrading would be, maybe more app control?
The app allows you to assign colors to each app installed on your device. The benefit is knowing if you have important notifications without turning on your device’s screen.
Different activities can be used like charging, low battery and so on. LightFlow has a test option in settings to see if it works for your device. Some devices will only use one color LED.
I can confirm that this does work on most higher end devices including Nexus devices.
LightFlow does not have much of an effect on my device’s battery.
TLDR; LightFlow is an app that allows you to change your device’s LED notification color to identify important notifications without turning on or unlocking your device.