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Blood, oxygenated or deoxygenated is always red due to the presence of iron in the blood. Veins carry deoxygenated blood. However, when seen through the skin, the veins appear to be blue. This is because, red has a small frequency and when light passes through the skin, the red color is absorbed and therefore our human eye sees it blue.
View through the thin layers of skin, and you can make out red streaks of vein too..
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