I have been noticing some dark colored sections in my stool; they are a light brown mostly except here's dark brown sections. and mucous. ?

Abnormal stools: Bowel movements' color changes may be from change in diet (from food colors), medications (e.g. iron or Pepto Bismol (bismuth subsalicylate) causing dark stools) or internal bleeding (dark like coffee grounds if from the stomach or red-brown if from the intestine). Seek medical attention in all cases, especially the latter which requires immediate attention as internal bleeding can be lethal.