Why do bruises change colour?
A bruise is blood spilt under the skin, and the colours are the stages your body's clean-up crew takes it apart in.
Simple intuition
The plain reason, in everyday words
Knock your shin hard enough and some of the tiny blood vessels under the skin tear. The skin itself does not break, so the blood has nowhere to go — it spreads out into the tissue and sits there. Fresh blood under skin looks red, then dark purple as it loses its oxygen. Your body cannot simply mop this up; it has to take the blood apart chemically and carry the pieces away, which takes a week or two. The colouring in blood is broken down in steps, and it just so happens that the halfway products are different colours. Green comes first, then yellow, then it is gone. So the sequence you watch on your leg is not the bruise healing in the sense of the tissue mending. It is a chemical demolition happening slowly enough to see.
The colours show the tissue repairing itself.
The colours track the disposal of spilt blood, which is a separate process from repairing the damaged vessels and tissue. The two happen at the same time but the colour is reporting only the clean-up.
You can tell how old a bruise is from its colour.
Studies of photographed bruises found the sequence too variable and observers too inconsistent for reliable dating. The only reasonably safe inference is that a yellow bruise is not brand new.
A bruise is blue because deoxygenated blood is blue.
Deoxygenated blood is dark red. The blue appearance comes from how skin scatters light, favouring the return of shorter blue wavelengths to your eye — the same reason veins look blue through the skin.
Rubbing a bruise helps it clear faster.
Rubbing can rupture more vessels and enlarge the bleed. Cooling early limits how much blood escapes in the first place, which is the only part of the process you have much influence over.
It is a rare case where an invisible biochemical pathway prints itself on your skin in order, at a pace you can watch over a week. It also carries a warning about evidence: a rule that felt obvious and got used in courtrooms for decades did not survive being tested properly, and knowing which parts of it did survive is the difference between a useful observation and a confident mistake.
Who worked it out
Bilirubin was isolated in the nineteenth century and its structure worked out by Hans Fischer, who won the Nobel Prize in Chemistry in 1930 for work on the chemistry of the same pigment family that includes haemoglobin.
What problem forced it
The enzymatic step that produces biliverdin was identified in the 1960s, when heme oxygenase was described, finally connecting the visible colours of a bruise to a specific catalysed reaction.
How it changed since
The pathway was later found to be inducible under stress and its products protective rather than merely waste, moving heme breakdown from a disposal story to a signalling one. In parallel, forensic medicine tested the old colour-dating rule and largely retired it.
Why veins look blue
The same light-scattering effect that misleads you about bruises misleads you about the blood in your wrist.
Newborn jaundice and blue-light therapy
The identical pigment, produced everywhere at once instead of in one spot, and a treatment that works by changing the molecule's shape with light.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.