Why do bruises change color?

A bruise forms when blunt force ruptures small blood vessels beneath intact skin, releasing blood into surrounding tissue. Fresh, that blood looks red or purple. As immune cells break down the hemoglobin, the iron-containing heme group is converted first to biliverdin, which is green, and then to bilirubin, which is yellow-brown, before the products are cleared away. The color sequence you observe over a week or two is a visible readout of that biochemistry, running roughly from red or purple through blue, then green, then yellow, then fading.

The same pigments explain other familiar colors. Bilirubin accumulating in the body from liver problems or from an immature newborn liver produces jaundice, the yellowing of skin and eyes, which is why bruise yellowing and jaundice share a shade. Deeper bruises take longer to appear because the blood must travel toward the surface, and gravity can carry it downward, which is why a thigh injury sometimes surfaces as discoloration near the knee days later.

Two clinical notes worth knowing. Bruising more easily with age is normal, since skin thins and capillaries become more fragile, and medications including aspirin, anticoagulants, and some supplements increase it. What warrants medical attention: bruises appearing with no remembered injury, unusually large or painful bruising from minor contact, bruising accompanied by frequent nosebleeds or bleeding gums, or bruises that do not improve over a couple of weeks, since these can indicate clotting disorders, platelet problems, or other conditions. And a bruise over a joint that will not bear weight, or one accompanied by significant swelling and deformity, is a reason to check for a fracture rather than to wait it out.