The core of clorophyll, the marvel behind the process of photosynthesis, consists primarily of Hydrogen, Carbon, Oxygen, and Nitrogen atoms arranged in a ring pattern. At the exact center of this complex molecule rests a single atom of magnesium- a dull-gray metal that gives much of the world its leafy-green color. Substitute that single magnesium atom for one of iron and presto!- a clorophyll molecule magically transforms to resemble closely the core of hemoglobin. Iron ore, another dull-gray metal, plays an important role in coloring the innards of all complex animals red. Green for plants; red for animals. So beautiful, so simple.
Some sea creatures—crabs, for example—use the organic molecule hemocyanin to oxygenate their blood. Remarkably similar to human hemoglobin, hemocyanin uses copper at its core instead of iron and that copper colors the blood green. Perhaps the reason that monsters in film and computer games seem always to have green blood.