why the human veins are red in colour?

 

In textbooks, arteries are often colored red while veins are colored blue. The colour given in books is to distinguish between oxygenated and deoxygenated blood for demonstrative purposes. In fact, all blood is red. Oxygenated blood is a brighter red while deoxygenated blood is a darker red color. Most veins carry deoxygenated blood, which is a darker color than oxygenated blood. The deep color of blood makes veins appear dark