What is the red pigment in blood

hemoglobin (or haemaglobin, if you're old fashioned). It's an iron based compound that binds oxygen.

It exists in two normal states. When it's carrying a molecule of oxygen, it's bright red.

When it releases its payload of oxygen, it changes shape to the deoxygenated form, which is dark red.

Haemaglobin. When bound to oxygen is bright red and when deoxygenated is blue due to change in conformation causing the molecule to absorb at a diffrent wavelength.

Iron.