I don’t have any direct experience with this coral but you are correct that since they are slow growers I would not recommend starting with a frag unless it’s showing the colors that you want. Genetic potential is difficult to quantify, (if even possible?) so it is not fair to assume that the seller will cut a frag from a “
Master”
That will eventually produce the color you hope for. Buy a properly colored frag and you don’t have to wonder.
If you can watch the frag being cut from the multi-colored master, then you have removed the potential for fraud but you have no guarantee that you will live long enough to see the colony grow into what you want. It may never happen, it may happen in a year or two, but what are you going to be thinking along the way if you don’t know what genetics came in the purchase?
HTH.