Red, blue, green and purple: the colours of…

The colours of…Christmas? After some long (long) time, I’m finally coming back with a (maybe) common topic, but it is worth to talk about.

But while you are reading, you will be wondering what the subject is, probably (if you haven’t done it yet) you will have (had) a quick look below to check a picture or some clues about that. Please, don’t do it. Let’s keep it a mystery. Could you guess the topic of the post? What are these colours referring to? Pigments, nature, planets… I’m afraid any of them, but not that far away. Here it is a clue: it is something related to animals. The colour of the eyes, tongue?  What if I tell you that these colours belong to the colour of the blood of different animals? You all know that human blood is red, despite the belief of princes and princesses to have blue blood (let’s see this later). What about the rest? Let’s keep reading.

Why is blood red?

This is the first question to answer if we want to understand the rest of the colours. You may have wondered why the blood is red, and some of you will probably already know the answer. Your first attempt to answer might be, because of the red blood cells. The blood is mainly formed by red cells, white cells and plasma. White cells and plasma are almost “colourless”, so that’s true, red cells give the colour to the blood. Then, the question would be, why are red blood cells red?

The responsible for the red colour is haemoglobin, a complex protein which transports oxygen, from lungs to tissues and organs. This protein is made of four subunits in which the haem group is placed somewhere in the middle. The haem group is a metal complex formed by an iron atom bound to a porphyrin. It can coordinate oxygen, to transport it through the body. The image shows the haemoglobin and heme group.

Haem group
So…why do veins look blue?

Blue is always the colour infographics and pictures selected for veins and red the one for arteries. Why? So as to explain this, we need to start with the basics: arteries transport oxygenated or clean blood whereas veins are responsible to carry deoxygenated or “dirty” blood. Let’s explain it in detail. When we breathe, we take oxygen which directly goes into the lungs. There, capillaries (tiny blood vessels) take this oxygen and transfer it to arteries. The heart pumps the “clean” blood to tissues and organs, oxygenating them, and veins receive the already-used blood. This blood that runs through the veins, carries CO2 (carbon dioxide), that we get rid off when we exhale. And this is one reason why pictures always show red and blue, to make that difference. But this is not the answer to the question. The deoxygenated blood indeed has lower levels of oxygen than the oxygenated one, but it is still red (running blood and wounds are not blue, are they?).

However, when you look at your veins, they look purple, blue or even green. The appearance of this colour is related to light. If they are blue, it means that the light colour reflected by the veins is blue and that they absorb the rest of the colours. Also remember, that veins are in the outside and arteries in the  inside, in the circulatory system. This is, veins are closer to the skin. Moreover, red light scatters much more than blue, and blue light is less tissue penetrating than red. So, gathering up all this information, this is what happens: (1) light goes through the skin till it reaches the veins. (2) Veins highly absorb red light and (3) mainly reflect blue light. Below, you can observe this process in a picture.

Skin absorption of light
Light penetration in skin

And this is a good summarizing video:

Different blood colours

After answering these questions related to human blood, I would like to ask you something: do you think all animals have the same blood colour? If your answer were no, you guessed it! There are lots of different blood colours, but the most common ones are those displayed in the next picture:

Blood colours in animals
Chemistry of the different colours of blood

And as you can see, those are the colours from the title. Let’s see why is the colour different, what is the functionality of these other colours and if this kind of blood can also transport oxygen.


Some other animals such as spiders, crustaceans, some molluscs, octopus and squids, have blue blood. In this case, their blood has a copper compound called haemocyanin. This complex is blue when oxygenated and colourless when deoxygenated. The horseshoe crab has this blue blood, which is so valuable that a quart of it can be sold for $15,000. Why is this happening? Their blood contains a special clotting agent, which is used for many pharmaceutical companies in vaccines. It could be a solution for COVID-19 vaccine, but what could be the consequences? Check National Geographic article.

If you want to get more information about this, I recommend you to watch the next video.


Some worms, leeches and marine worms have green blood. This is due to chlorocruorin, an iron complex similar to haemoglobin. However, this compound is not found in the cells, unlike haemoglobin. It is green when oxygenated and could turn red in higher concentrations.


Brachiopods, penis worms and peanut worms have purple blood. A compound called haemerythrin gives the purple colour when oxygenated, although it is colourless when deoxygenated. This compound is not as efficient as haemoglobin at oxygen transport.


Finally, there are some animals with yellow blood such as beetles, sea squirts and sea cucumber. The pigment that gives this colour is haemovanabine, a vanadium-based compound, whose function of this compound is not oxygen transport, although it remains still unknown.

As you can see, red is not the only colour of blood. Now, when you hear about blue blood and princess, I am sure you will think differently.

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