Photosynthesis doesn't cease under pure green light - the rate is simply extremely slow. Additionally, there are literally hundereds of known accessory pigments in plants which have no known function other than support as antennas. This study which I read recently shows that a species of green algae responds well to green and red light (around 520 and 660 nm respectively) for resetting the circadian clock (the perception of day and night periods). So, while I believe a green light will have little if any noticeable effect on your cannabis, I believe that any light will have some effect.