Bile is green because it contains the green pigment biliverdin, which is formed by a breakdown of hemoglobin. Biliverdin is then converted into bilirubin in the liver.
The biliverdin pigment can cause vomit to appear in a range of colors from bright yellow to dark green, depending on how much bile is being regurgitated. Biliverdin is also the cause of the greenish color that can be seen in some bruises on the skin. An excess of biliverdin or bilirubin, or both, in the circulatory system and tissues causes the hepatic disease jaundice, which is characterized by a yellowish pigmentation of the skin.