The elusive green flash is hard to photograph because it is (1) rare and (2) brighter than the surrounding area so that it usually comes out overexposed and colorless, even with spot metering. But because the brightness is variable you cannot, except by luck and eyeball judgment, find the right exposure. After many failures to photograph the sun's image turning green just before it disappears, Jef finally got a few with decent, if not perfect, exposures.
These two shots were taken only a few seconds apart as the sun set. The second image is the magnified central image from the first image, you can see the effects of the contrail of an airplane that had just flown through.