Hi, Kinesis, welcome. There's lots of 5mm green led lights. They are small enough to be very convenient on your keychain. KevinL sells some even smaller 3mm ones but IMO those are TOO small. Unless you want tiny just for the sake of tiny, I'd go for a 5mm model.
I think KevinL's keychain lights have only white or red leds, but the leds are easy to replace. lsdiodes.com sells leds in different colors and sizes.
If you don't mind ordering from overseas, Dae sells some nice "Fauxtons" (Photon clones) in different LED colors:
link. You have to buy ten of them but they're not expensive (85 cents each). I'm not sure if all 10 have to be the same color.
Countycomm.com also has some with green leds but I don't like their design quite as much.