What causes the difference in the color of blood

Oxygenated (arterial) blood is bright red, while dexoygenated (venous) blood is dark reddish-purple. The difference is color results from the electronic state of the iron ion (ferrous vs ferric), which in turn influences the π → π* and n → π* electronic transitions of porphyrin and hence its optical characteristics.

Pigment molecules binding to oxygen molecules.