Bilirubin is a yellow pigment which arises from the catabolism of red pigment haemoglobin of old and worn out RBCs. The pale yellow colour of blood plasma is largely due to the presence of bilirubin. It has to be excreted into the bile and from there it is excreted out of the body along with the faecal matter. If, all of the bilirubin can not be excreted from the body then the skin and mucous membranes assume a yellowish hue, giving rise to the condition called jaundice.