Nope, not really. You could see light beige through to dark brown (through experience only - I don't make a habit of studying this !). I suspect it was something they ate before coming to you. Carry on with the pig nuts and make sure that they are bright and healthy and make sure there isn't any blood in the doings.
If the colour doesn't return to normal after a day or two chat to your vet.
Cheers
Gavin