When logic state is FALSE, LED become dark and change color when it's TRUE (can emit small amount of light).
There could be 3 different colors: Red, Yellow, Green

Simple example

LEDs show if we have enough items in chests:
1st chest -> Red = critical low amount
2nd chest -> Yellow = low amount
3rd chest -> Green = enough amount
It also could be used to build led display with logic gates and all that cool stuff

Right now I'm using Lamps but they have 2 problems:
- not working during day
- with no energy they have blinking warning icon