Why do bruises change colour?

A bruise forms when small blood vessels under the skin break and blood leaks into surrounding tissue. At first the pooled blood looks red or purple-blue because of oxygen-rich and oxygen-poor haemoglobin. Over the next few days, white blood cells called macrophages break the haemoglobin down into biliverdin, a green pigment. Biliverdin is then converted into bilirubin, which is yellow, and finally the iron is stored as brownish haemosiderin before the body clears it. Most bruises fade completely within about two weeks, though they heal more slowly on the legs and in older people.