Most of these flashes have a perfectly good explanation. The earth's atmosphere acts like a prism and disperses the light of the sun into a spectrum, and the last portion of this spectrum that one usually sees is green.
Conditions have to be just right, though.
A flat horizon and a layer of warm air are important. But even when conditions seem perfect, the green flash does not always appear, and all one sees is the red tip of the sun sinking out of sight. A. Young, San Diego State University, reported at a recent meeting of the American Astronomical Society on a peculiar type of optical illusion that can afflict observers of the setting sun. It seems that the intense red light of the sun near the horizon can bleach the red-sensitive receptors in the retina so that an intent observer becomes temporarily color blind to red. The true green flash may never have appeared at all.