The fabled green flash happens sometimes when the atmosphere refracts the sun's light in just the right way at sunset. I watch the sunset every single night that I'm at Paranal, unless there's something I have to be doing at that moment, and usually the sun sets with no hint of green. People on the platform usually say something about how they see it much more often at La Silla, but I didn't see it there either.
Tonight, I didn't see the flash by eye, but I caught it on camera nevertheless. A strip of green floated above the rest of the orange disk, too faint for me to notice it but there without any doubt.