A respiratory pigment is a molecule, such as hemoglobin in humans and other vertebrates, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin. Hemoglobin is bright red when oxygenated, and dark red(purplish) when deoxygenated, oxygenated haemocyanin is blue in color, deoxygenated it is almost colorless. Oxygenated chlorocruorin turns from green to red, whereas oxygenated haemerythrin is a violet to pink colour and colorless when deoxygenated. Any of various colored conjugated proteins, such as hemoglobin, occur in living organisms and function in oxygen transfer in cellular respiration.