A bruise is a record of a small leak. A knock can damage tiny blood vessels without breaking the surface of the skin. Blood escapes into surrounding tissue, and the body gradually clears it away. The changing colours are a visible trace of that clean-up, although the skin above the blood, the depth of the injury and the light in which we look also affect what we see.¹
It is easy to imagine a bruise following a tidy timetable: red on day one, blue on day two, green on day three. Real bruises are much less orderly. Understanding the chemistry explains why colours change; it does not let us read an exact date from them.
What is under the skin?
Small vessels called capillaries can tear when tissue is compressed or struck. The escaped red blood cells remain below the surface, so a bruise is different from a cut that bleeds outward. Pain and swelling may accompany the leak, while the coloured patch becomes visible through the layers of skin. A larger or deeper injury can look different from a small, shallow one.¹,²
The blood's most recognisable colour comes from haemoglobin, the oxygen-carrying protein inside red cells. Near the skin, newly escaped blood may look reddish, purple or dark. It need not look bright red: colour depends on where the blood lies and on the tissue through which light passes before reaching our eyes. On darker skin, bruises may be seen as dark brown, purple or black, and later colours may be less obvious than a textbook illustration suggests.²,³
A useful comparison is a coloured object viewed through layers of translucent paper. Adding layers changes the colour and contrast you perceive without changing the object underneath. Skin is more complicated than paper, but the analogy helps explain why identical pigments need not produce identical-looking bruises in different people or body sites.
A small chemical recycling operation
The body cannot leave the escaped red cells in place indefinitely. Cells involved in clearing damaged tissue break down haemoglobin and process its iron-containing haem group. In that pathway, biliverdin is a green pigment; an enzyme then converts it into bilirubin, which is yellow. These products help explain why green and yellow can appear as a bruise resolves.⁴
The sequence is a chemical pathway, not a rule that the whole mark must turn a solid green and then a solid yellow. Different parts of the leak can be at different stages of clearance. Blood can sit at different depths, and more than one pigment can contribute to what is visible at once. A bruise may have a purple centre and a yellowish edge, or may fade without a clearly visible green stage. The appearance is the result of chemistry and optics.⁴,⁵
Other products of red-cell breakdown, including iron-containing pigments, can contribute brownish tones. Eventually the material is removed or recycled, and the discolouration fades.⁴ The colour change is therefore a sign that the body is processing blood outside its vessels, not that a fresh dye is being added under the skin each day.
Why colour cannot tell you the exact age
If purple tends to appear earlier and yellow later, why not use a colour chart as a clock? Because the starting conditions vary. Bruises differ in size, depth and location; people differ in skin tone and in how readily they bruise; even lighting or a photograph can change what an observer calls the colour. Two patches from the same incident may not look identical.
The evidence is especially important when the timing of an injury matters. Reviews of bruise assessment warn against assigning a precise age from colour alone. One review concluded that the available research was too limited and inconsistent to make colour the primary basis for dating a bruise. Another found that even when some colours suggested a bruise was not recent, visual assessment had major limitations.⁵,⁶
A study of children with accidental bruises found that physicians' estimates of bruise age were highly inaccurate within a day of the actual injury time. The authors also reported substantial variation between observers.⁷ That finding does not mean every observation about a bruise is useless. It means the familiar colour sequence should not be turned into a stopwatch, especially for clinical or legal decisions.
Think of a banana changing from green to yellow to brown. The general progression is real, but the colour of one patch does not tell you the minute it was picked. Bruises have even more biological variation, and unlike a banana they are seen through skin. The analogy is imperfect, but it separates a broad process from an exact calendar.
What affects whether a bruise stands out?
The force and site of an injury influence the amount of blood that escapes. Ageing skin may be thinner and less cushioned, making bruising more noticeable after a minor knock. Medicines that affect clotting can also change the likelihood or extent of bruising. An NHS trust notes these factors while cautioning that bruises may appear bluish-purple, brown or black depending on usual skin colour.³
Most ordinary bruises fade as the blood is cleared. But a tendency to develop unexplained or unusually frequent bruises is a different question from the colour of a known bruise; clinical guidance recommends discussing unexplained bruising with a healthcare professional.¹,² This article cannot determine the cause of a particular mark.
The answer to the colour puzzle is that the body is dismantling and removing leaked blood. Haemoglobin, its breakdown products and the skin above them create the shifting palette. The broad movement from darker tones towards green, yellow or brown can be familiar, but no single colour reliably stamps a bruise with its age.