Now that is a green bulb, But it would dull on cloudy days and it will be useless at night when it is needed the most.

Unlike "western" people, they do sleep at night, so do not need the light. But they do not have the light even during the day.
Problem is, then in these areas the windows able to withstand the monsoon are not affordable. And such trick with the bottle makes really the difference there - the required hole is quite small, so not so difficult to seal and even when the bottle degrade soon, it is easy to replace. And the material is readily available...

You could add something phosphorescent in

When i was a kid, i bought few packs of those glow in the dark plastic stars, inserted them all on a stick (spaced few mm from each other) so that it looked like a string isolator on a high volt line, and it was a GREEN lantern - it is charged by light and emitting green light. It was so huge that you actually could see a bit with its light

Then i wanted to make it more compact, so i ended up with shredding the stars to pieces and putting them in a clear plastic box, which made it much brighter when staring into it, but i dont remember whether it improved or reduced light output

You could, but for the people there such stuff is EXTREMELY expensive, so they better go sleeping at night and wake up early to use the daylight time as much as possible...

Arent there natural substances (easily accessible)  that are phosphorescent ?

For sure not without at least some processing. And that is out of the reach there...
Even if those "glow in the dark" toys do not cost much compare to e.g. gasoline, these people would rather spent their hard earned money on the gasoline they really need for their business, then for some tiny glow for the nighttime.

Phosphorescent ? And why no fireflies, like in movies !

Like said Medved, the best thing to do when come the night is sleep, what they can do by night ? Watch tv ?
If these people went light i'm shure these would rather find wood or other crap to make a fire

This is too cool! They cannot even keep up with the demand there isn't enough bottles! I think it would still work on cloudy days because the water magnifies the light from all directions...?

Even when the amount of light fall to 1/4, it does it's job...

 :oThats incredible!

I like it because it is simple,economical, and has a cool white look to it.