I have yellow diarrhea every 3 days or so. The more fats you eat, the more diarrhea and yellow color you will see - the color means the fats in your diet are not being digested because your gallbladder is not providing enough bile to digest the food.
My gasteroenterologist said, on our first visit, "Yellow stool? Okay, let's do the CCK HIDA scan for the gallbladder." And the scan showed a non-functioning gallbladder.