Here’s what Professor Hallux has to say:
Ouch! That’s going to leave a bruise! If you’ve had one yourself you know they can turn almost all the colours of the rainbow as they heal, and it’s down to one thing – blood!
The bang or the bump breaks the tiny blood vessels under your skin. The blood pools in the tissue and starts off looking a bluey/purply colour. Now your blood isn’t blue – it’s bright red – but because you’re seeing it through your skin – the red tones are lost and it seems much darker.
Within one or two days, the bruise can seem almost black – that’s because of how the haemoglobin is changing as the bruise heals. After a few more days as the blood is broken down and removed from the area, the bruise becomes lighter, turning to green or yellow and then faint brown.
Most bruises are pretty short lived and disappear in a week or two. But always tell your parents if you notice a bruise that just won’t heal.