Why Do Bruises Change Color?
A bruise shifting through a rainbow is not healing. It is a chemical cascade: trapped red blood cells break down, and the pigment changes step by step from hemoglobin (red) to biliverdin (green) to bilirubin (yellow) to hemosiderin (brown).
Right after impact it looks red. By the next day it turns purple. After a few days, blue, then a tinge of green. Finally it goes yellow, then brown, then disappears.
Why does a bruise shift through a whole rainbow?
The wound is probably healing, fading over time. The gradual softening looks like recovery.
It is not healing.
A bruise is a small leak. Capillaries beneath the skin have ruptured, and blood has seeped into the surrounding tissue. The trapped red blood cells have nowhere to go, so they break down right there, and that chemical reaction is what changes the color.
The initial red is hemoglobin in its natural color. As oxygen leaves, it darkens into a deep purple, then blue.
Immune cells called macrophages begin dismantling the red blood cells, and hemoglobin converts to biliverdin, a green pigment. Biliverdin then becomes bilirubin, a yellow pigment. Finally hemosiderin remains, a brown iron-storage form. Even that iron is eventually reabsorbed.
The color change is not recovery. It is a cascade of pigments transforming step by step.
On the left is the color showing through the skin; on the right is the molecular cascade happening underneath. Drag the day slider (0 to 14) and the color advances automatically from red to blue to green to yellow to brown, while the molecule diagram highlights which pigment is at work in that stage. Use the surface/molecular button to switch between the two views, and the five color-stage buttons to jump to any point in time.
Use the day slider (0 to 14) to watch a bruise advance from red to blue to green to yellow to brown, and switch surface/molecular to see the pigment cascade underneath (hemoglobin → biliverdin → bilirubin → hemosiderin).
Reading the days from the colorRed and purple suggest day 0 to 2, blue and black day 2 to 5, green day 5 to 7, yellow day 7 to 10, and brown day 10 to 14. These are rough guideposts only, since timing varies a lot between people and body parts.
Easier to bruise with ageAs we age, blood vessels and skin grow more fragile, so even small bumps bruise more easily and linger longer. A big mark from a light knock is not necessarily a sign of trouble.
When to check with a doctorIf a bruise lingers past two weeks, appears with no impact at all, or keeps growing, there may be another cause, and it is worth checking with a doctor.