Have you ever bumped your shin on a coffee table, forgotten about it, and then found a purple patch days later? Over the next week or two, that same mark shifts through a surprising range of colours: dark purple, then greenish, then a yellowy fade before it disappears.

It's easy to think of a bruise as damage that simply wears off. But those colour changes are actually a progress report. Each shade marks a specific chemical step as your body takes apart trapped blood and saves what it can. Once you know what you're looking at, a bruise stops being a blemish. It becomes a window into one of your body's quiet clean-up crews.

Just beneath your skin runs a dense web of tiny blood vessels called capillaries. Their walls are thin, only a single cell thick in places. That makes them perfect for delivering oxygen, but not great at surviving a knock. When something hits you hard enough, these vessels tear while the tougher skin on top stays intact.

With nowhere to escape, the blood seeps into the surrounding tissue and pools there. At first that blood is still rich in oxygen, so a fresh bruise often looks red or pink. Within hours, the trapped red blood cells use up their oxygen. Haemoglobin, the protein inside them that carries oxygen, darkens as it does. Swelling and tenderness arrive too, as your body sends fluid and immune cells to the scene.

The blue-purple look is partly an optical trick. Your skin acts like a filter. Red light travels deeper and gets absorbed by the dark pooled blood, while blue light scatters back towards your eyes. It's the same reason veins look blue even though the blood inside them is dark red. A deeper bruise often looks more blue, while one close to the surface tends to look redder or more purple.

Takeaway

A bruise is a leak behind a closed door. The damage happened beneath skin that stayed whole, so your body has to clean up entirely from the inside.

Once blood has leaked where it shouldn't be, your immune system treats it as debris. Specialised clean-up cells called macrophages, a name that roughly means big eaters, move in and start swallowing the stranded red blood cells one by one. Inside these cells, the real chemistry begins.

Haemoglobin contains a ring-shaped structure called haem, which holds an iron atom at its centre. Macrophages use an enzyme to crack that ring open. This releases the iron and turns the rest of the molecule into biliverdin, a green pigment. That's why a bruise often turns greenish after a few days. A second enzyme then converts biliverdin into bilirubin, which is yellow. As bilirubin builds up, the bruise fades to yellow or yellowish-brown.

Bilirubin doesn't stay put. It travels in your bloodstream to your liver, which packages it into bile and sends it out through your gut. This same pigment helps give stool its colour. The shades of a bruise rarely move in neat stages, though. Different parts are cleared at different speeds, so you might see purple in the centre and yellow at the edges at the same time. That's also why doctors can't reliably date a bruise just by its colour.

Takeaway

The colours of a bruise aren't decay but careful demolition. Each shade is a different stage in taking one molecule apart, piece by piece.

The iron freed from haem is far too valuable to throw away. Iron is the part of haemoglobin that actually grabs oxygen, and your body has no efficient way to get rid of it, so it holds on tightly. Macrophages store some of it in a protein called ferritin, and some as a brownish compound called haemosiderin. That's why an old bruise can leave a faint brown stain that lingers for weeks.

Gradually, the iron is handed to a carrier protein called transferrin and transported to your bone marrow, where new red blood cells are made. This isn't a special trick reserved for bruises. Every day, your body retires billions of worn-out red blood cells and recycles most of their iron the same way, mainly in the spleen and liver. A bruise is simply that everyday routine happening somewhere you can see it.

Most bruises clear within two to three weeks. Bruises on your legs often take longer, partly because blood pools there more easily. It's worth talking to a doctor if you bruise often without remembering a bump, if bruises are unusually large, or if they come with nosebleeds or bleeding gums. These can point to low platelets, a clotting problem, medication effects such as blood thinners, or certain vitamin deficiencies. Thinner skin in later life also makes bruising easier.

Takeaway

Your body runs a closed-loop recycling system for iron every single day. A bruise just gives you a rare chance to watch it working.

A bruise tells a short story in colour. Red and purple mean blood has leaked and lost its oxygen. Green and yellow mean immune cells are breaking haemoglobin down. The fade back to normal means the iron has been reclaimed and the rest shipped out.

To help it along, use a cold pack during the first day to limit swelling, and rest and raise the area if you can. And watch for patterns. Frequent or unexplained bruising is worth a conversation with your doctor.