I don't believe the lines are compatible, meaning you can't breed a vpi axanthic to a joliff axanthic and get axanthics, do it wouldn't be necessarily be selective breeding. I believe it's just that each line is a different morph that, genetically, doses practically the same thing. So if you get a snake from a line that tends to keep better color, you'll get a better colored snake, and if you get one from a very brown line you get a very brown snake. I've only seen one axanthic in person and I don't know what line it was from duo I can't really tell you what line stays greyist.