L.Z.
I found this to be the case with both my girls when they were teething - diarrhea is a common side effect of it. Also, he could be adjusting to the new cereal. Are you giving him fruits mixed in or just plain? It could happen either way, and the yellow color is most likely because of the solids. It sounds like it's both things happening at one time! When my youngest, now almost 12 months, was eating baby food and cereal, she had loose stools often, but since she's been on all "people" food, it has been much more solid (I know, it's very graphic!!). I wouldn't worry, but as always, if you are concerned and it's not going away, call the pediatrician! Good luck!