The common
misconception that blood which lacks oxygen turns blue probably
comes from the fact that veins appear blue and blood in the veins is
typically heading back to the lungs, hence depleted of oxygen.
People who perpetuate this myth often claim that the reason we never
see blood in its blue form is that the instant we get cut, the blood
is exposed to oxygen and instantly turns red. However, when you get
blood drawn from your veins that isn't exposed to air, it is dark
red.
When blood is deprived of oxygen it actually just turns dark red.
When it’s oxygenated, it turns a brighter red. The red color
primarily comes from the hemoglobin, which contains four heme
groups. These heme group’s interactions with various molecules end
up giving it the dark red or light red color we see. The hemoglobin
itself is a protein that binds with oxygen to be distributed
throughout the body in blood.
Veins are very close to the surface of skin. This location under the
skin is largely why veins appear blue despite the fact that the
blood is dark red. This is from the way light diffuses in the skin.
Veins appear blue from the way subcutaneous fat absorbs
low-frequency light. This permits only high frequency blue and
violet wavelengths to penetrate through the skin to the vein, with
the other wavelengths getting filtered off from the pigmentation of
your skin.
If a person has darker or lighter skin the veins tend to appear
green or brown. People with extremely light skin, such as an albino,
will typically have veins that show up as dark purple or dark red,
more closely resembling the actual color of the blood running
through the veins.