Why does my skin turn colors when I get hurt?

In all the soft tissues of your body there are many tiny blood vessels, the capillaries. When you get a bruise, some of the capillaries may be broken. Then red cells of the blood leak out and collect in the tissue underneath the skin. That patch of red cells and broken-down red cells seen through the skin may look blue or even black depending on how many red cells are trapped there.

Fortunately for you, your body can repair its cuts and bruises. So the usual treatment for a bruise is to just patiently wait for the slow job of repair to take place.