A respiratory pigment is a molecule, such as hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin

Hemerythrin

Hemerythrin is an oligomeric protein responsible for oxygen transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm, magelona. Recently, hemerythrin was discovered in methanotrophic bacterium Methylococcus capsulatus...

 and chlorocruorin

Chlorocruorin

Chlorocruorin is an oxygen-binding hemeprotein present in the blood plasma of many annelids, particularly certain marine polychaetes. Its affinity for oxygen is weaker than that of most hemoglobins...

. Hemoglobin is bright red when oxygenated, and dark red(purplish) when deoxygenated, oxygenated haemocyanin is blue in color, deoxygenated is almost colorless. Oxygenated chlorocruarin turns from green to red where oxygenated haemeryhrtin is a violet to pink colour, and colorless when deoxygenated. All vertebrates use the hemoglobin respiratory pigment.

.