Your veins appear blue mainly because of how light interacts with your skin and the blood inside the veins. Although blood is always red—bright red when oxygenated and darker red when deoxygenated—the blue appearance of veins is an optical illusion. Blue light, which has shorter wavelengths, is reflected back to your eyes by the skin and veins more than red light, which penetrates deeper and is absorbed by the blood. This reflected blue light makes veins look blue to the naked eye, especially in individuals with lighter skin or thinner skin where veins are closer to the surface.
Other factors influencing the visibility and color of veins include skin tone, age (thinning skin makes veins more visible), body composition (less fat means veins are closer to the surface), and genetics. Sometimes, veins can appear green instead of blue due to variations in light absorption and reflection through the skin.
While blue veins are usually harmless and a normal part of the body's circulatory system, if they become painful, swollen, bulging, or are accompanied by skin changes, these could be signs of conditions such as varicose veins or other vascular problems that may require medical attention.
