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The explanation of the phenomenon is as follows: When the sun is low the violet blue light on the long path that the light rays cover through the lower dusty layers of the atmosphere are virtually entirely diffracted from the beam of sunlight.
The yellowy-orange light is largely absorbed by the water vapour in the humid layers of air. In the direct beam of the sunlight this then only leaves the colours of blue-green and red.
Now upon refraction in the atmosphere a slight dispersion occurs, as a result of the fact, that blue light is broken up more than red light. The apparent raising of the disk of the sun caused by the refraction (the lifting amounts at the horizon to around half a degree) is then a little stronger for blue-green rays than for red rays. As a result of this one can see two disks of the sun that largely overlap. The blue-green disk is slightly higher than the red one. Because of this when the sun sets, a green segment remains at the end. The relatively only rarely observed green flash occurs if the top of the sun is divided into separate segments by mirages. At sunset each segment finally takes on a green colour. The circumstances for the occurrence of the green flash are most favourable if the air is transparent and the sun, also close to the horizon, is whitish in colour. It is of course also possible that a green flash will also be seen at sunrise.
source NTT De Zee April 1981