Normal Brown colour of stool is due to

Hemoglobin from RBC undergo metabolism and produces bilirubin and biliverdin. This, in turn, conveed into urobilinogen and stercobilinogen.This urobilin and stercobilin are responsible for giving colour to urine and stool respectively.During obstructive jaundice absence of this stercobilin leads to clay colored urine(Ref: Vasudevan, textbook of biochemistry)