This is a sunset. It displays the fleeting phenomenon know as a "Green Flash".

Nobody's got a single answer as to why they appear, but the two prevailing ideas are that both atmospheric refraction and retinal bleaching by the sun's overwhelming brightness conspire to create the green flashes of color shown here.

I've never seen one, despite knowing about them and searching during multiple tropical trips, where they're supposedly more prevalent.

Here's the gallery where the image seen here came from, there's more.

This is a good page on the whys and hows of green flashes. Get looking.