Consider the classic color representation of arteries as red and veins as blue. Why do we use these colors?

Question

Consider the classic color representation of arteries as red and veins as blue. Why do we use these colors and what specifically do they describe? What portion of the circulatory system does not follow this model and why?

Answer 1

Arteries are represented by red because the blood in them  is high on O2 and has a bright red color. Veins are blue because the blood in them is low on O2 and has a dark red color and they appear blue beneath the skin.

The portion of circulatory system that does not follow this model is pulmonary circulatory system in which pulmonary artery brings the darker blood with lower levels of O2 to lungs and pulmonary veins brings the lighter blood with higher levels of O2 back to heart.