You will want the purple side up. GSP spreads by encrusting whatever it is on or near in the purple stuff, this is actually the coral "skeleton". So you will want the purple up, also think about it now as to what you want covered in GSP. It will cover any rock it can have contact with over time. If you isolate it to a rock out on the sand bed you can control it a little easier as it can be broken off the sand easier than it can a rock. It has been considered by some to be a weed as it grows fast.