SPS corals frequently have brightly colored tips or whole branches in the wild. These colors tend to be shades of blue, purple or pink. It is common, unfortunately, that the hobbyist brings home a brightly colored SPS coral, places it into his tank only to have the color fade to a boring brown. This phenomenon is sometimes referred to as brown-out.

The reason why the coral has bright colors to begin with has been a hot topic of discussion. Most experts in the field chalk the colors up to an adaptation to protect themselves against the relatively high levels of UV radiation that the corals receive on the shallow reefs. Similar to how a persons skin suntans to protect itself against the sun. Optimum SPS coloration also appears to track with general coral health. Increased coloration is often associated with increased polyp extension and growth rates. The brightly colored areas are often sites of new growth. In the aquarium, there appears to be a number of other factors as well that can affect SPS coloration. These include:

Alkalinity level appears to be a critical factor in SPS coloration. Normal seawater level of 2.5meq/l does not seem to provide for optimum coloration. Alkalinity should be maintained in the range of about 3.2 – 4.0 for best SPS coloration.

Lighting intensity also appears to be a factor. The general rule is that more intense lighting equals better coloration. This might possibly be due to the fact that more intense lighting also generally has a more intense UV component to the lighting. Although some people seem to be able to achieve good coloration with less bright VHO or PC lamps, most SPS hobbyists opt for metal halide lighting which provides for the most intense lighting possible. There has been a general trend to put more and more lighting over reef tanks in general over the last few years. Two 400W MH lights over a 70 –90 gallon tank is pretty standard for SPS. Four to six 400W lamps over a 200-300 gallon tank is also pretty common. Using the old watts/gallon rule, most SPS tanks have 8-12 watts/gallon.

Lighting color spectrum affects coloration in a couple of ways. The color of light directly affects how the color of the coral is perceived even though the color remains unchanged. A coral with purple tips under 10K lighting may look blue under 5500K lighting for instance. The other aspect is that lighting color can apparently affect overall coloration of the SPS coral. Typically, the higher color temperature lamps seem to provide for better coloration. For that reason, most SPS keepers opt for 10K lamps or higher when using MH. VHO users typically supplement with actinic lamps to achieve a higher color temperature.

Calcium levels have, I believe, an indirect affect on coloration. Good calcium levels promote coral health and growth with tends to optimize coloration. Calcium levels in the range of 400-500 are generally considered acceptable.

Salinity level of the water may also be a factor. Although I have not come across this cause and effect in the literature, I have observed this in my own tank. My main tank underwent a period of neglect while on vacation and shortly thereafter. I began to notice that many of my SPS were starting to brown-out. The only water parameter I could find out of whack was that the salinity had crept up to about 1.027. I decreased this back down to 1.025 over a couple of days and within a week, with no other changes, the coral coloration returned to normal.

Temperature may also have an affect according to some peoples observations. If so, this might be related to the normal temperature range for the specimen in question.

Further Reading:

Coloration in Acropora nano By Dana Riddle and Andy Amussen
Experiments with alkalinity and lighting and its effects on  the coloration in Acropora nano

Pocillopora- The Cauliflower Coral and the beginnings of coloration pt. 1 by Eric Borneman

Bird's Nest Coral..Feathers Not Included ..and coloration, part 2 By Eric Borneman

The Elusive Blue Tipped Acropora By Eric Borneman