Their diet always has been high quality frozen foot (various LRS "frenzy"s, rod's food, PE mysis) + selcon soaked nori and selcon soaked pellets combined with whatever algaes they pick at in the tank. And they share the tank with a wild caught Kole Tang (my first tang from 5 years ago before I switched to only CB) that has perfect coloration.
So if it is diet, then boy you really gotta work hard to get them up to the perfect coloration!
I am saying something might be missing from their diet before you purchase them since they come with the HLLE already. Perhaps it is already too bad to completely fix by the time they are purchased.