1. What a bruise is: blood leaked from damaged vessels under the skin
A bruise is blood that has leaked from damaged blood vessels and pooled under the skin. The changing colours reflect how the pigment called haemoglobin inside the leaked red blood cells is slowly broken down [1].
A fresh bruise looks deep red because the haemoglobin still has oxygen attached to it. As that oxygen is passed to the surrounding tissue and decreases, the colour moves towards a dark blue or purple [1]. It is not that the blood is blue, but that it becomes darker as oxygen levels drop. The way light reaches through the skin also affects why it looks blue-purple, so this explanation alone does not cover everything.
2. Why bruises turn green and yellow: immune cells break down haemoglobin
The cells that come to clear away the clump of leaked blood are immune cells, such as macrophages. Enzymes inside these cells break down the haemoglobin [3].
The part of haemoglobin that creates colour, called "heme", first changes into a green substance called biliverdin. This is quickly reduced to yellow bilirubin [2]. Green and yellow are explained as colours that appear during this breakdown process. Japanese medical information sites also state that bruises change from red, blue, or blackish colours to green and yellow, becoming less noticeable [4].
3. Why bruises end up brown: iron left in the tissue, after about 2 weeks
The iron part of haemoglobin is trapped in the tissue in the form of haemosiderin, which is orange to golden-brown [2]. After the green and yellow fade, this iron remains, making the bruise look brownish [3].
Most bruises are said to disappear in about 2 weeks without treatment. Some guides suggest that yellow and green are prominent around 5 to 10 days, while yellow-brown to light brown appears around 10 to 14 days [5]. However, the number of days varies greatly depending on the size of the bruise, its location, and the person [3]. These numbers should be taken as "roughly" rather than exact rules.
4. Why even experts find it hard to tell a bruise's age from its colour
There is a general flow from red to blue-purple, green, yellow, and brown. However, forensic medicine explanations state that colour changes are not linear and do not progress as predicted [6].
A systematic review published by UK researchers in 2005 examined studies on whether the colour of a child's bruise could determine its age. Of 167 studies, only 3 met the criteria. In those, the rate of correctly identifying the age within a 24-hour margin did not reach 40%. When distinguishing between new, medium, and old bruises, the accuracy was only 55 to 63% [7]. The timing for yellow to appear also varied between studies, ranging from 24 hours, 48 hours, or within 48 hours [7].
This review concluded that there is no scientific basis for estimating the age of a bruise from its colour, and it should be avoided in child protection procedures [7]. The subjects were children, so it cannot be said for certain that the same applies to adults.
5. Why bruise colours can look different from person to person
For people with darker skin, a fresh bruise may not look red or blue, and the sequence of colour changes may not match what is shown in books or diagrams [3]. This is introduced as being influenced by factors such as the position of pigment within the skin.
Diagrams showing colour changes are only "common examples". They are not a pattern that fits every body exactly. It is natural for the appearance to differ, and this does not indicate superiority, inferiority, or abnormality.
6. When to talk to an adult or a doctor about bruises
Japanese medical information sites state that if bruises appear suddenly in many places, heal slowly, grow larger, or are accompanied by swelling, heat, or pain, there is a possibility of an underlying illness, so it is a guide to consult a medical institution [4].
If you are worried about the colour or spread of a bruise, do not judge it yourself. Tell an adult, and if necessary, have a doctor look at it. If you have worrying symptoms, it is recommended to consult a parent or doctor.
7. How you can investigate your own bruise's colour changes
As something you can do today, if you have a bruise, try writing down the colour in words when you notice it, after the pain has settled. If an encyclopedia lists the order of colours, comparing them might help you feel that "colour changes are not linear". Because there are individual differences in appearance, being out of sequence does not mean you are outside the standard.
If you want to read the sources, the fdoc explanation in Japanese is easy to read [4]. The 2005 review is in English and is published on PubMed Central [7].