Basically the light is broken up into individual colors, causing a green shimmer of light just as the sun sets. According to NASA, the illusion is typically only visible from places with a low, distant horizon. A green flash lasts only for a second or two.
The best places in San Diego to see a green flash are Point Loma near the Old Point Loma Lighthouse and Sunset Cliffs, though green flashes have been reported at Ocean Beach. These areas provide that low, distant horizon previously mentioned.