When you do an image search in Google for "Bankers Lamp", you will end up with a bunch of lamps with green shades and brass bases that look pretty much alike.
Why are they all green? Well, in doing a bit of research, I came across an interesting discussion on just this topic. It seems that no one in this forum that I was reading really knew the answer. There were some guesses about how the green filtered the light so that it was not such a strain on your eyes and things like that, but it was all just guessing.
So, I did a bit of thinking and came up with my own guess.
I bet you're just dying to know what it is, right? Well, I did some research on depression glass and found out that there were several manufacturers of glass during the depression, when Banker's Lamps were very popular. They made glass that was very affordable so that even in those lean years, everyone could have glass. They mainly made things in blues and greens. This is in keeping with the bankers lamp designs.
Many people don't realize that the early bankers lamps were mostly blue. Although green became the color of choice in following years. I'm betting it was because those colors were the cheapest to produce and were able to be sold to the public during those harsh economic times.
So, that's my guess. What do you think? Could I be right? Let me know if you've got the answer.