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When light falls on the skin surface, it penetrates the skin, is absorbed and re-emitted. The veins present underneath the skin surface re-emit small amounts of red light and high amounts of blue light, which is visible. This is one of the reasons, why veins appear blue.
When the blood carries oxygen, it is known as oxygenated blood (carried through arteries). After it is supplied to the organs and comes back to the heart, it is known as de-oxygenated blood (carried through veins). And as the oxygen levels falls (de-oxygenated blood), the colour changes from red to dark red, which in turn appears closer to blue.
when light falls on the skin, this small difference in colour is amplified and veins tend to appear blue and arteries that have smaller diameter and located deep inside the skin look red.
the visual reception process. For example, if there are red coloured and purple coloured objects, you brain perceives the purple as a shade of blue. So your brain processes whatever you visualise through the retina. So this can be one of the reasons why veins appear blue to you even though they are not.