Even though it looks purplish when seen at certain angles and places, blood in the veins is very much red. The easiest way to confirm this is to look at your blood the next time you have a blood test. Blood is always extracted from veins, so this is a foolproof test. You should see that your blood, although darkish, is very red.
You might still argue that veins look blue or green or purple or something other than red. Indeed they do. I never refuted that. What I did say is that the blood in the veins isn’t blue. The reason veins appear blue is something else altogether.
Veins are located deeper under the skin than arteries. Due to this, light has to travel a greater distance through the skin to reach veins. During the course of this extra journey, certain wavelengths of the light are absorbed more easily as compared to others. In this case, the red and yellow wavelengths find it more difficult to reach the veins. Because of this, the light reaching the veins is left with primarily green and blue wavelengths. When this light emerges from the skin and reaches an observer’s eyes, the observer sees a greenish-blue colour.
The myth that blood in the veins is blue has been perpetuated in many ways. The biggest culprits are probably biology textbook authors, who conveniently and irresponsibly colour all veins in their books blue. This aids easy identification of veins and arteries in diagrams, but it leads to severe misunderstandings in the students’ minds. By looking at the diagrams again and again without being corrected by their teachers, the students naturally assume that the blood in the veins is blue.
Interestingly, some animals do have blue blood. Most of these animals are slimy creatures that are not very pleasant to look at, so I shall leave out the pictures. If you really want to see them (trust me, you don’t), you can do so here. The reason that these animals have blue blood is that they haven’t developed the red iron-based pigment haemoglobin like most advanced animals have. Instead, they have to make do with a copper-based pigment, and this gives them their unusual (but rather cool) blood colour.