It is due to a pigment called uricobilin or urobilinogen which is a derivative of bile pigments.

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The yellow color in urine is due to chemicals called urobilins. These are the breakdown products of the bile

pigment bilirubin. Bilirubin is itself a breakdown product of the heme part of hemoglobin from worn-out red blood

cells. Most bilirubin is partly broken down in the liver, stored in the gall bladder, broken down some more in the

intestines, and excreted in the feces (its metabolites are what make feces brown), but some remains in the

bloodstream to be extracted by the kidneys where, converted to urobilins, it gives urine that familiar yellow tint.