Veins

Human blood in veins is not actually blue. Hemoglobin give blood its red color. Deoxygenated blood has a deep red color, and oxygenated blood has a light cherry-red color. The misconception probably arises for two reasons: 1) Veins below the skin appear blue or green. This is due to a variety of reasons only weakly dependent on the color of the blood, including subsurface scattering of light through the skin, and human color perception. 2) Many diagrams use colors to show the difference between veins (usually shown in blue) and arteries (usually shown in red).