The color of blood depends on different chemicals that special proteins of the blood contain. Because some of the chemicals change color while reacting with oxygen, the color of blood is revealed when oxygenated.
We know at least 5 colors of blood running through blood vessels of various animals.
1) Red. Most vertebrates including humans have red blood because of the iron in our plasma.
2) Yellow. Beetles, sea squirts and cucumbers have yellow blood because of vanabin, containing vanadium and the functional aim of which is still unknown.
3) Green. Many annelids like leeches, marine and segmented worms, have chlorocruorin in their blood that turns green when exposed to oxygen.
4) Blue. Spiders, crustaceans, octopuses and squids, mollusks have copper in their blood, which in reaction with oxygen becomes blue.
5) Purple. To transport oxygen brachiopods and some worms use hemerythrin, which, when oxygenated, turns violet-pink.
