Why are arteries red and veins blue?
Neither are red or blue; the blood that passes through them is brighter red in arteries and a deeper maroon in veins because of the difference in oxygen dissolved.
Veins are the only blood vessels that can be seen near the surface of the skin since arteries are generally present at deeper levels.
The light incident upon skin tends to show veins as bluish because of the different absorptions of the red and blue wavelengths. This does not reflect the actual colour of the vessels.
Within the vessels, arterial blood is a brighter red because of the colour of oxy-haemoglobin present in the erythrocytes.
Blood in the veins has lost the oxygen and this de-oxygenated blood is a deeper red, almost maroon.
In models and drawings, arteries and veins are depicted as red and blue for distinction.