Just read through this interesting article in regards to GFP and color morphs in corals in response to light intensity or photostress and blue light intensity.
Photodamage of symbiotic algae exposed to thermal stress is involved in mass coral bleaching, a major cause of reef decline. Photoprotection is therefore a v...
www.frontiersin.org
There’s a lot of anecdotal reasons why corals may change their coloration; light intensity, nutrient imbalance, or lacking essential amino acids, but i think reading a few articles on coral pigmentation might help deepen our understanding on the mechanism, and thus, the root source of observable color shifts.