What is the red pigment that gives blood cells their colour?

The red colour of blood cells is due to the presence of a protein called haemoglobin. Haemoglobin is an iron-containing protein that binds to oxygen molecules and transports them through the bloodstream. The oxygen molecules bind to the iron atoms in the haemoglobin, causing them to change shape. This change in shape causes the haemoglobin to turn from red to a dark red colour. The blood appears bright red when it is carrying oxygen and dark red when it is carrying carbon dioxide.