Why do bruises change colour?

The weird and wildly coloured bruises/contusions we all get, sometimes rooted in our less-than-graceful moments, change their colours due to lack of oxygen.

Bruising begins when a trauma or injury crushes the little blood vessels under the skin. Blood pools and turns red at the site, inflammation and localized pain may occur. Within a few days, hemoglobin (the compound that makes our blood red) imbues the injury with black, blue or purple colours. As days goes by, the pooled blood is absorbed by your body, the oxygen level in hemoglobin decreases causing colour changes which ultimately fade and disappear. The process usually occurs within two weeks, however, certain medical conditions may prolong healing.