The colour of a bruise changes with time and is due to the disintegration of the red blood cells. There is hemolysis and breakdown of the hemoglobin into the pigments hemosiderin, hematoidin and bilirubin by the action of enzymes and histiocytes. A fresh bruise is red in colour. This is due to oxy-hemoglobin. Within 1 day, the colour changes to blue i.e. deoxy-hemoglobin. In 2-4 days, the colour becomes brown i.e. hemosiderin. It then becomes green in about 5-7 days. This green colour is due to hematoidin. Then the bruise becomes yellow in 7-10 days which is because of bilirubin. The colour then fades and the skin regains its normal colour in around 15 days. This is because the pigments have been removed by the phagocytes. Thus the colour change is red,blue,brown,green and yellow. An exception to this is a subconjunctival hemorrhage which changes colour from red to yellow directly. This is because the hemoglobin is being constantly kept oxygenated by the air during degradation.