Why do bruises go yellow?
Bruises occur when blood is released from the capillaries of the circulatory system and is trapped under the skin. It is the hemoglobin in the red blood cells and the breakdown products of the hemoglobin that cause the coloration seen in bruises as they age. Most of the color seen in bruises is due to the biochemical breakdown of hemoglobin, which is the oxygen-carrying component of blood. There are two major breakdown products that can lead to visible color in a bruise. These are a green pigment called biliverdin and a yellow-brown pigment called bilirubin. Bruises usually start out as a dark blue or crimson; fade to violet, green, and dark yellow; then turn a pale yellow; finally disappearing completely. Bruise coloration is used by some forensic pathologists to help specify the times of some events, but the sequence of the color change is not that reliable.
