Why do veins appear blue?

I’d like to straighten out an incorrect statement that I made in a previous post and in my book on color.

On page 156, I said that in the region surrounding the lips, “there are relatively more veins carrying blue deoxygenated blood,” It turns out to be a misconception that blood turns blue when it loses its oxygen contact.

Yet veins deep below the skin certainly do appear blue. Why, then has no one seen blue blood? I had always assumed the answer was that if when a vein is cut open, the blood immediately turns red on contact with air.

In fact, blood is always red (or at least a deep maroon color) when it is deoxygenated.

What’s going on here? The scientific answer involves a lot of factors, but according to Wikipedia, on light Caucasion skin at least, “veins appear blue because the subcutaneous fat absorbs low-frequency light, permitting only the highly energetic blue wavelengths to penetrate through to the dark vein and reflect off. A recent study found the color of blood vessels is determined by the following factors: the scattering and absorption characteristics of skin at different wavelengths, the oxygenation state of blood, which affects its absorption properties, the diameter and the depth of the vessels, and the visual perception process.”

READ MORE
PDF of scientific paper
Wikipedia on Veins
Good summary by Quora.com
Science Blogs.com on the subject  
Color and Light: A Guide for the Realist Painter
Previously on GJ: Three Color Zones of the Face

Photo from We Heart It.
Thanks, Myke!