Your rods are used for night vision.
Rods can't see red light, that's where the use of the red light came from........
With green light, your cones (at least 2 out of 3, maybe all 3) and rods can see. Plus you need less green light to see with than red light. So a dim green light (dim enough not to screw up your night vision) is easier to see with at night.