What color is a bruise right after you get one? It's usually dark purple or some shade similar to that. Turn over your hand and look at your wrist, what color are your veins? Pretty similar to the color of a new bruise, right, dark purple? Even though we know that blood is flowing through the veins, they don't appear red, but dark purple-blue. This is partly because the blood that flows closest to the surface of your skin is in veins and veins generally are carrying blood back to the lungs for re-oxygenation. Blood that has low oxygen content will appear less red and more dark purple. But, it also has to do with the tint your skin gives the blood that makes it look purple.

Anyway, so back to the bruise question...A bruise is really an injury to tiny blood vessels. The impact that causes the bruise has broken some of the tiny blood vessels in your muscle. These blood vessels have leaked a little bit of blood into the muscle tissue. This is why the bruise first appears dark purple, like the blood in your veins.
Gradually, your body begins to heal the bruise. The blood is broken down and reabsorbed, so the dark purple color is slowly removed. As the blood is broken down it changes color too, so that's why a bruise can change color over time.

I think the blood that causes a bruise in the tissue gets slowly broken down, so it starts out purple, which is sort of blood colored. Then the heme molecule in hemoglobin in red blood cells gets broken down into bilirubin, which is yellow.That's all I know. You could check out
www.google.com and search for bruises or something to see if you can find anything else. A lot of it is just first aid stuff, though.