Why Do Bruises Change from Blue-Purple to Green and Yellow?

A bruise is not skin that has been “dyed blue”. An impact ruptures small blood vessels, allowing blood to leak into tissue beneath an otherwise intact surface. At first, haemoglobin and its oxygenation state, the depth of the blood and the person’s skin colour combine to produce red, purple or blue-black appearances. What we see is light passing through skin and being absorbed and reflected by these materials.

The local clean-up process then begins. Macrophages ingest red blood cells, and the haem in haemoglobin is broken down first into greenish biliverdin and then into yellowish bilirubin; iron is also recovered or stored. Several pigments may exist in different areas at once, so a bruise does not change colour as a neat, uniform sequence. A blue-purple centre and yellow-green edge can appear together.

This chemistry shows that the bruise is being cleared, but it is not a reliable clock. Injury depth, the amount of bleeding, body location, skin colour, lighting and individual differences all affect appearance. Systematic reviews have also found that a bruise cannot be dated accurately by visual examination or a photograph alone. Yellowing broadly indicates more haemoglobin breakdown products, but it cannot identify the particular day on which the injury occurred.

https://medlineplus.gov/bruises.html
https://pmc.ncbi.nlm.nih.gov/articles/PMC1770270/