I would probably use white towels... thats what I use if a piggy is poorly, or after operations, it is just less likely that any bedding gets into potential wounds etc, and you can see if there is any oozing or bleeding. Now with regard to pregnancy, I would say that you would probably want to keep a close eye on your sow for a few days after giving birth, so I would say some sort of very light coloured towel or fleece would be your best bet, then you can see if she is bleeding when she shouldnt be anymore.