Between fleas a possible staining from dirt is the only thing I can think of. When a flea dies it goes from plump to thin because it loses the blood inside of it, just like a tick if you squeeze the body the blood it has sucked out pops out. So it's possible that during flea season the staining is from that, but that tends to lighten up after a bath and look more white. Without seeing her in person it's hard to tell. I do know that a Pyr's color will change, but yellowish all over isn't what they change too.