(bruises) are common skin discolourations that occur when small blood vessels (capillaries) near the skin's surface break from the impact of an injury, causing blood to leak into the surrounding tissue. Biological Process

When a blunt force impacts the body, it may not break the skin, but it can rupture the delicate capillaries underneath. This trapped blood initially appears as a flat or slightly raised mark. Over time, the body’s immune system breaks down the blood and reabsorbs it, which causes the bruise to change colour. The Colour Cycle

Around day 5–10, the body begins breaking down haemoglobin into biliverdin .