Sometimes we trip over or hurt ourselves in other ways. When it happens some of our blood vessels break, blood piles up under our skin and we can see this as a bruise. These nasty things have the familiar ‘black and blue’ or purple appearance in the beginning but gradually change into different colours.
The purple colour is given by haemoglobin, a protein that carries the oxygen in our red blood cells. Our body reacts to this with some white blood cells called phagocytes ‘eating up’ the materials in the bruise. As the phagocytes degrade the haemoglobin, they turn it into other molecules. As you can imagine, different molecules show different colours and the bruise will change colour with time to green, yellow and brown. When everything has been cleared up by your immune system the bruise disappears and you’re ready to bump into something else.
Answered by Science Museum Explainer José Monteiro.