As far as I understand, JD is still working on figuring it out. I haven't heard an official update that he couldn't replicate it. He has had others in his collection with the same what he calls "Dirty", where they start off clean looking but over time they develop darkening, much like a cow retic, or spots on CG/banana to a lesser extent. We know SK Axanthic is part of the equation, but other than that we do not have a definitive answer on anything. I don't think he has even managed to prove out "Dirty" as a morph, which is understandable since you can't even see it at first.