There are plenty of very colorful things to see on a natural reef with sunlight. Admittedly the predominant color is brown because that's the color of the zooxanthellae (symbiotic algae in the coral). Some of the most beautiful corals (in my opinion) are the non-photosynthetic corals (particular Dendronephthya or carnation coral), unfortunately some of these are very difficult to keep in a reef aquarium.
