A bruise occurs when small capillary blood vessels break under the skin. Hemoglobin in this leaked blood gives the bruise its classic purplish hue. The body then ropes in white blood cells to repair the damage, causing hemoglobin to break down into biliverdin, which is green, and then bilirubin, which is yellow. The debris at the bruise site ultimately clears, and the color fades.—From New Scientist