One of the chief biliary pigments is bilirubin. It has a red-orange colour, and is derived from the decomposition of haemoglobin. It can be oxidised first into a green, then into a blue, then into a red, and finally into a yellow-brown pigment. Between the blue and red a violet substance is produced, but it is uncertain whether or not this is only a mixture of the blue and red pigment. The formation of the green pigment, according to Städeler, is due to oxidation, together with the addition of a molecule of water.