I used to think that green on black was the way to go because of that whole "ow, bright light, pain, my eyes!" thing, but it turns out that you can avoid the pain of bright screens and extend your battery life by just turning down the screen brightnesss. I keep mine as low as it goes while still having the backlight on. Though, in a well-lit room, it is possible to read black text on white with the backlight off as well, thus hyper-extending battery life.