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When white light hits the skin, red light can travel further below the surface than blue light. Its longer wavelength reaches deeper where your veins are located. From there, hemoglobin –– the protein that makes your blood red –– absorbs the red light.
Blue or green light, on the other hand, doesn’t travel as deeply. They are more easily deflected from your skin than red light. So when white light hits your skin, blue and green light isn’t absorbed in the same way as red light. The result is that, despite blood always being red, you ultimately see blue veins or green veins.
https://www.usaveinclinics.com/blog/why-are-veins-blue-and-green/
