Why is blood red, and are veins really blue?

Red blood cells are packed with hemoglobin, a protein containing iron at the center of a ring structure called heme. That heme group absorbs and reflects light in a way that appears red, and the exact shade shifts with oxygen binding: oxygen-rich arterial blood is bright scarlet, while oxygen-depleted venous blood is a darker brick red. Both are unmistakably red. Anyone who has had blood drawn from a vein has seen deoxygenated blood, and it is dark red rather than blue.

The blue vein appearance is an optical effect in the skin. Light penetrating tissue is scattered and absorbed differentially by wavelength: red light penetrates deeper and is largely absorbed by the blood, while blue light scatters more in the upper layers of skin and is reflected back to your eye. The result is that a vein a few millimeters below the surface appears blue or green even though the blood inside is dark red. Depth matters, which is why superficial veins can look green while deeper ones look blue, and why the effect varies with skin tone and thickness.

Blue blood does exist elsewhere in nature, which is part of why the myth persists. Horseshoe crabs, octopuses, squid, and many crustaceans use hemocyanin, a copper-based oxygen carrier that genuinely appears blue when oxygenated and colorless when not. Some marine worms use chlorocruorin and have green blood, and certain lizards in New Guinea have green blood from a bile pigment. Horseshoe crab blood has real medical importance, since a clotting agent derived from it is used to test injectable drugs and medical devices for bacterial contamination, though synthetic alternatives have been developed to reduce reliance on the animals.