Bruises Change Color Because They’re Losing Oxygen
A bruise is caused by bleeding under the skin; tiny capillaries (blood vessels) are crushed, which expel blood that’s trapped under the skin. Initially, the bruise will just look red because the blood is still oxygen-rich. Within one to two days, the blood begins to lose its oxygen, turning purple.
Then, after three or more days, bruises will start to turn green, yellow, or grey thanks to compounds called biliverdin and bilirubin that break down the hemoglobin to absorb the “good stuff” (such as iron) for the body to use. The rest of the waste is eventually purged from or absorbed by the body.
