Cecil says you can’t photograph a Green Flash with an ordinary camera. Actually, you can.
If you look up Green Flash in Wikipedia, one of the photos used to show the effect was taken by an ordinary SLR camera in Santa Cruz, California at the beach over-look behind the Abbott Lighthouse.
I’m a guide there and, if there is clear weather, you are likely to see a green flash more often than not. As Cecil mentions, sometimes it’s a blue flash and, rarely, a violet flash-- and often it’s more than one, a series of flashes one after another, depending on the instability of the air at sunset.
When the sun is low and it’s possible to look at it directly without frying your eyes, watch for distortions on the edge of the sun as it sets through layers of air of different temperatures. The distortions will work up to the top of the sun and a small blob of the sun will float off and flash as it dissipates. And, as said above, there may be just one or a series of flashes depending on the air that evening.
Almost every evening at sunset I set up a spotterscope and provide binoculars for people passing by so they can see the effect. They are seldom disappointed. We are usually rewarded with flashes that can range from “not tonight,” to muted by haze, to brilliant emerald green, to sapphire blue, to a deep violet.
I actually have a sort of Green Flash Society going with some local people stopping by to watch the effect as a group. Some take pictures. The image is small, but with a zoom lens and computer you can get a really nice, enlarged image.
Also, as an added bonus, after sunset you can turn around and watch the Belt of Venus rising over the Santa Cruz Mountains to the east. (Again, see Wikipedia.)
There is a curse involved, however: Once you’ve seen the Green Flash you’ll never watch a sunset again. You’ll be so busy looking for the flash, you won’t spend the quiet time watching the sunset.
It’s the price you must pay for seeing this beautiful event.
(Note: I just returned from Costa Rica and was unable to see the flash there. The air was too stable every night and no flash developed. Disappointing.)
Ah, Santa Cruz!