Blood appears a different colour in arteries and veins because hemoglobin's colour depends on:

1. The colour difference comes from one molecule. 2. Arterial and venous blood carry different amounts of oxygen. 3. Hemoglobin is a pigment that changes color. 4. The colour depends upon the degree of oxygen saturation. 5. So the difference reflects oxygen saturation rather than the vessel itself. _Source: OpenStax Anatomy and Physiology 2e (CC BY 4.0), Ch 18 'The Cardiovascular System: Blood', sections 18.1-18.6_