Respiratory pigments are organic compounds that consist of metallic copper or iron that binds oxygen. These pigments may be in form of solution within the blood or body fluids, or they may be present in specific blood cells. These pigments respond to a high oxygen concentration by combining with oxygen and to low oxygen concentrations by releasing oxygen.
 The four most common respiratory pigments are hemoglobin, hemocyanin, hemerythrin, and chlorocruorin.

Hemoglobin
Hemoglobin is a reddish pigment that contains iron that bind to oxygen.
It is the most common respiratory pigment in animals and found in all vertebrates. Hemoglobin may be carried within red blood cells or simply dissolved in the blood or coelomic fluid.

Hemocyanin
Hemocyanin contains metallic copper and has a bluish color when oxygenated, and always occurs dissolved in hemolymph. Hemocyanin is the most commonly occurring respiratory pigment in molluscs and certain crustaceans.  Hemocyanin  release oxygen easily and to provide a ready source of oxygen to the tissues as long as concentrations of oxygen in the environment are relatively high.

Hemerythrin
Hemerythrin contains iron and is pink when oxygenated. It is nucleated cells, rather than free in body fluids or hemolymph. Sipunculans, priapulids, a few brachiopods, and some polychaetes have hemerythrin.

Chlorocruorin
Chlorocruorin also contains iron but is green when associated with low oxygen concentrations and bright red when associated with high oxygen concentrations. Chlorocruorin occurs in several families of polychaete worms.