From what i understand the Pigment in a coral is different than the Zooxanthellae, the pigment will get lighter or darker depending on the amount and quality of light it is exposed to. Morelight in the 420 nm range will have a tendancy to bring out a particular corals innate abiity to phluoresce. so a dark purple specimen under 10k up high may become light pink because the higher light wavelength does not excite the phluorescent pigments as much.. at the same time a light pink may become dark purple for the same exact reason.... Zooxanthellae react to light in generall,, the more light generally the denser ? is that a word? the Zoo is which can make a coral appear darker in coler and more healthy looking.. along those same lines ,even though low nutirent water is best for Acros to survive in,, those tanks with readable amounts of nitrates (under 10 ppm) seem to have a darker coloration to them as opposed to undetectable readings. .. these are just my opinions and experiences. and are by no means considered undeniable fact.. thank you and good night