system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some bruises turn yellow or green after their purple phase?
gpt
The black / purple stage of a bruise is blood soaked into the tissue under the skin, due to ruptures in the capillaries.

Oxygenated blood is the red colour you're used to seeing.

Deoxygenated blood is darker and lacks the red, so you get the purple colour.

As the bruise heals the first thing that happens is your body absorbs the iron in the hemoglobin.  The rest of the red blood cells are broken down into biliverdin (which is greenish), which is in turn broken down into bilirubin (which is yellowish).  

Because the process is consecutive you all ways get the same sequence of colours, though how quickly it happens will vary.