My healthy 18 mo old has had pale yellow stools for months. Liver tests were normal. What other tests need done? Ped isn't concerned. Should I be?

Normal baby poop. The first stools a baby makes are black meconium. These gradually change to that yellow color. As time goes by, then stools will evolve into the brown color you recognize as normal stools. If you have already been to the doctor, and there is no concern with blood testing; then i would suggest that you adopt a relaxed attitude and let time take care of this one for you.