that could be from something you ate, like salads etc.... Or it could be that your food is going thru your body to fast, and you are not getting the benfits from your food. Or the old wives tale says it could be that you have a cold. I would call your Dr. if you have it again. Hope this help, I have the rainbow of feces for about 3 weeks.
Did you drink anything purple lately? I have had green stools before when I drank purple gatorade and koolaid. Scared me until I realized what caused it.