Corals have calcium carbonate skeletons that the coral polyps secrete. When the coral is alive and healthy, it looks colorful instead of white.
The color of many corals comes from proteins in the corals themselves, others come from the colors of symbiotic algae. The algae, or zooxanthellae, live in coral polyps, have a golden green appearance, and convert energy from sunlight into food for their coral host to grow and survive. Fluorescent proteins (or FPs) in corals can appear red, yellow, green, and blue, and are thought to help coral animals cope with temperature stress, as an ultraviolet shield. In fact, scientists have observed that different color corals of the same species have different thermal tolerance.
In honor of International Year of the Reef, Pantone selected “Living Coral” as 2019’s Color of the Year. Living corals come in many colors, not just pink. For instance, this species, Stylophora pistillata can be pink, blue, or green. It seems like this particular colony was green, based on Dr. James Dana’s 19th century illustration.