A medical explanation for the rainbow of colors that occurs in skin bruises
The colors of a bruise appear courtesy of proteins being broken down inside the white blood cells. The first color is purple, which is caused by the hemoglobin being broken down and losing its oxygen. That happens immediately after the injury.
The first breakdown product of hemoglobin is called biliverdin, which appears within a few days of the contusion and explains the greenish tint. The green biliverdin is then converted to a yellow molecule called bilirubin. That process takes anywhere between a few days and several weeks, depending on the size of the bruise. Once bilirubin is made, it dissolves in the blood stream and is carried off to the liver and kidneys for final processing and excretion.
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