Why does a bruise change colour as it heals?
The colour sequence is your body dismantling the haemoglobin that leaked out of broken blood vessels, and each shade corresponds to a different breakdown product.
A bruise forms when blunt impact ruptures small vessels under intact skin. Blood escapes into surrounding tissue with nowhere to drain, so it must be broken down and cleared in place by immune cells.
The usual progression:
Red, in the first hours. Fresh blood, still carrying oxygenated haemoglobin.
Blue, purple or black, roughly day one to five. The trapped blood loses its oxygen. Deoxygenated haemoglobin is darker, and the way light scatters through skin makes it read as blue-purple.
Green, around day five to ten. Immune cells begin breaking haemoglobin down. An intermediate compound called biliverdin is genuinely green.
Yellow or brown, around day ten to fourteen. Biliverdin converts to bilirubin, which is yellow, then to haemosiderin, which is golden brown. These are cleared last.
The bruise then fades as the final products are carried away.
Why bruises sometimes move downward is gravity: pooled blood tracks along tissue planes, so a knock to the thigh can surface at the knee.
Why some people bruise more easily — thinner skin and more fragile capillaries with age, some medications including anticoagulants and aspirin, and certain supplements. Skin tone affects how visible each stage is; the same process occurs regardless.
When to seek medical advice: bruises appearing with no remembered injury, very frequent or unusually large bruising, bruising alongside bleeding gums or frequent nosebleeds, a bruise that keeps growing or becomes hard and increasingly painful, or any bruise with a suspected fracture. Bruising around both eyes or behind an ear after a head injury needs urgent assessment.
This is general information, not a substitute for seeing a clinician.