It is the red blood cells degrading. A bruise is leaking of blood into the damaged tissues, and as the hemoglobin (red) degrades, the heme pigment also degrades, changing the color from purple to various other shades, like green and yellow. Bilirubin (the ultimate breakdown product) is a greenish-yellow color, so the degree of bleeding and timing of the degradation dictate what color your skin is at any given time. Eventually, the damaged tissue is healed, and the red blood cells are eaten up, leaving no visible mark.