Stercobilinogen (fecal urobilinogen) is a chemical created by bacteria in the gut.

[1] Bilirubin is a pigment that results from the breakdown of the heme portion of hemoglobin.

Urobilinogen / stercobilinogen is colourless and is further oxidised to stercobilin which imparts colour to feces.

Darkening of feces upon standing in air is due to the oxidation of residual urobilinogens to urobilins.

Stercobilinogen is absorbed and excreted by either the liver or the kidney.