It could also happen if you changed your curing racks and having your soaps touching any exposed metal including stainless steel. I have had this happen on the edges if I accidently have a soap touch the edge of my rack. I line the shelves of my racks but the edges still have exposed stainless. As for lighting I only get color changing after my soaps are at market exposed to sun from the UV. UV is filtered when coming through glass.
