Some shallow water coral have symbiotic relationships with algae called zooxanthellae. The algae use the coral as shelter and protection while the polyps receive nutrients from the algae that photosynthesize food from sunlight. This algae helps corals to quickly create large reefs and even provide some color to the corals, such as greens, brown, and reds. Other colors the corals themselves produce, like blue and purple.
Deep-sea corals, though, do not have algae because there is no sunlight in the deep sea, so many of these species are white. There are still some colorful varieties, though! Remember, some corals produce their own color. Deep-sea corals grow much more slowly than their shallow water cousins and tend to grow more individually than as a reef.