that is a weird question and funny thing is I bet what ever the reason for starting and staying with green is the culprit as to people doing Hightech style movies (Matrix, Equilibrium etc.) have chosen to overcast them in green (the matrix in particular because they wanted the actual matrix scenes to be green cast to give the feeling that you were inside a computer) I have one theory and that is that maybe the reason is because the boards use copper and the green is such a sharp contrast so as to allow assemby by humans to be easier on the eyes to find the openings. What is funny is that that mask is how I intend to generate a sort of PCB etching system that is automated using very low wattage lasers to remove the material to allow the etchant to work. then once the copper is gone the same lasers will drill out the fiberglass board to make the holes.

BUT why should it be green? is it because it was the standard then or cheapest that time? how about white or black -sure these neutral color should be cheap.
My guess is that, green dye could surface the gold copper circuits more visibly and clearly for the electronic components to be attached and mounted soon after the PCB is ready for further manufacturing.