I think lighting. I have had many purple/yellow SPS turning green when first introduced to a new tank...After acclimating to the environment then the color comes back if the conditions are right...Sometimes even better color than in previous tank...
Correct just lighting.
Many say that a weekly dose of some iron will do the trick to keep the yellow but never worked in my system.
My acros that are yellow stay yellow just to keep them close if not over the 350 par.
No3 between 5ppm and 10 ppm, Po4 at 0.06.