The green flash is a rare phenomenon that occurs just at sunset, usually over water. Just as the last bit of sun disappears on the horizon, there is a second where the higher light frequencies dominate and the sun changes from its red-orange color to green. A very few have reported seeing blue or violet flashes as well.
For the rest of us who have not seen this, the green flash is real. Transoceanic airline pilots have almost all seen it and it has even been photographed.