Dunno if you read this before but I was just doing some reading and came across this at reefkeeping.
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Myth 5: The "K" rating of aquarium lamps plays an important role in the coloration of corals.
Myth 6: Corals are colorful because of their symbiotic zooxanthellae.
Zooxanthellae are golden brown to deep brown in color, depending on their pigment content and the light/temperature environment in which they are found. The bright colors in corals arise from either animal-based or skeleton-based pigments. Many gorgonians, soft corals, and a few stony corals incorporate pigments into skeletal elements. Others have animal based pigments that are either biosynthesized or acquired through diet. Many zooxanthellate corals have their bright colors because of a family of multi-hued fluorescing proteins. These pigments seem to be produced in response to a given light environment. The primary control on their production appears to be total irradiance level, and little evidence exists to suggest that the "k" rating of bulbs will influence their production. There is also a strong genetic component, although the specific aspects of fluorescing proteins and their respective genes have not yet been worked out. The color temperature of light bulbs most likely influences the perceived color of corals in a tank, with ultraviolet components enhancing highly fluorescent pigments. Certain bulb temperatures may have enough of their spectrum skewed in relative distribution that total irradiance with a given wattage may be affected, and thus total irradiance influencing the relative production of fluorescing proteins.
It is notable that some pigments appear to be formed as a result of low light, while others appear to be formed in environments with high irradiance levels. For more information, see the article and references located here.
Potential: varied. If a brightly colored coral is producing fluorescing proteins in response to low light, placement in high light environments may result in bleaching. Other aspects of fluorescing protein production may be related to bleaching resistance in high light environments. Otherwise, coloration is largely aesthetic for reef aquariums.
Distribution: widespread. It is becoming more widely recognized in aquarium circles that coral coloration does not arise from the colors of zooxanthellae. However, many articles in the lay press still wrongfully propose this notion. The belief that the k-rating of bulbs is important to coral coloration is widespread among the hobby populace, and appears to be a common misconception in all countries
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Might not give you any DIRECT info on your specific prob, but maybe shows why colouration changes in corals...
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