Zero's nose changes a bit, too. Well, the pink half does, anyway. If he's really relaxed, sleeping, etc., it's usually paler, but it gets redder, more flushed if he's playing or being crazy. After a long session with Da Bird or the laser pointer, it's nearly red. His pink toes also change a bit, too.
I think it's mostly just heart rate & blood pressure.
I think it's mostly just heart rate & blood pressure.