WHY does a bruise appear and changes its color?

When a bump or bruise occurs, the capillaries are damaged. Blood flows from them into the surrounding soft tissues under the skin. Blood contains plenty of hemoglobin, giving it that bright red color.

White blood cells arrive at the site of the injury. They surround the site of the hemorrhage and begin to destroy the blood cells.

Destruction of hemoglobin produces decomposition products — biliverdin (green bile pigment) and bilirubin (yellow-red bile pigment).