Have they recently had reddish foods (carrot, red lettuce and cabbage, beetroot, or red-tinted chew toys)? This frequently affects the colour of their poo. Unless you notice any other worrisome symptoms (blood, loss of appetite, suspect the piggy is in pain), I wouldn't worry.
If concerned, you can take a picture of the poop and show it to us, or bring the piggy to the vet just in case.