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.
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.
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:
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.
BLUE
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.
GREEN
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.
PURPLE
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.
YELLOW
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.
Why yellow and blue don’t make green
You will find images like the one above, that show that red, yellow and blue are the primaries and that yellow and blue make green.
Sometimes this is represented as a colour wheel:
So some people say yellow and blue make green. And you will find other answers that say that yellow and blue make black. How can this be?
Well, we need to understand a little science to get to the bottom of this.
The figure below shows what happens when you mix an ideal yellow dye with an ideal blue dye. The blue dye reflects light perfectly in about a third of the spectrum (and absorbs perfectly in the other two thirds). The yellow pigment reflects light perfectly in about two thirds of the spectrum (and absorbs perfectly in the other third).
The problem here is that the blue and yellow pigments (between them) absorb perfectly across the whole spectrum. The people who say that yellow and blue make black are saying so because of this argument.
Note that blue is a particularly bad choice of primary because it absorbs so broadly across the spectrum. [Making the blue even purer would only make the problem worse by the way.] Yellow is a good choice of subtractive primary because it only absorbs in one third of the spectrum.
The problem is, the people who say that blue and yellow make black are wrong of course. Every child knows this. In practice, if we measure the reflectance spectra for blue and yellow pigments they don’t look like those ideal ones I showed above. For a start, they are quite smooth. Here is a reflectance spectrum for a real yellow pigment. (The reflectance factor, by the way, is the proportion – or per cent – of light that the colorant reflects at each wavelength.)
Notice that with a real yellow colorant, it does not reflect perfectly in the middle and long wavelengths and it does not absorb perfectly in the short wavelengths. It reflects and absorbs to some extent all the wavelengths but it absorbs more at the shorter wavelength and absorbs at less the middle and longer wavelengths. The same is true of a real blue colorant; it does not absorb perfectly at the middle and longer wavelengths. The consequence of this is that you don’t get black if you mix blue and yellow. You would get black if the pigments were ideal but they are not. We live in the real world. However, you certainly don’t get a lovely bright green as shown in the colour wheel with red, yellow and blue primaries. You would get a dark desaturated murky dirty greenish colour. The main reason for this is that the blue is absorbing too broadly. Interestingly, if you look at the artist John Lovett’s page he explains that to mix a yellow and blue you should use a yellowish blue (and a bluish yellow).
Now let’s see what happens when we mix cyan and yellow dyes. We’ll start with the ideal colours.
It’s very nice. We get a lovely green colour. Cyan is a great subtractive primary because unlike blue it absorbs in only one third of the spectrum (the red or long wavelengths). Note that it is precisely because the cyan does not look pure that makes it a great primary – that’s why I get so furious about people saying the primaries are pure colours. The cyan looks bluish-green because it reflects in two thirds of the spectrum and only absorbs in the reddish part. Neither the cyan nor the yellow dye absorb in the middle (green) part of the spectrum and therefore the result of mixing cyan and yellow is a lovely green. Except it is not quite true. Remember, this is for ideal pigments. Real dyes do not look like that. Refer back to the measured reflectance spectrum for the real yellow pigment. In reality cyan and yellow do make green but the green might be a little less saturated than you may wish for because of the unwanted absorptions by the two dyes in the areas of the spectrum where ideally they would not absorb. (It was the great Robert Hunt, who worked for many years at Kodak – for those who knew him – who taught me about unwanted absorptions.)
Have you ever seen this happen. Of course, you have. Whenever you use a printer (which typically uses cyan, magenta and yellow primaries) to get a green, the printer is using cyan and yellow to make the green.
Remember those people who say that you can’t make blue because – yawn – it’s a pure colour that can’t be made by mixture? Well, have you ever printed out blue on a printer? Of course, you have. Let’s look again at our ideal primaries and see if we can explain it.
That’s right. Mixing cyan and magenta makes blue. The cyan absorbs in one third (the red third) and the magenta absorbs in one third (the green third) but neither absorb the short wavelengths.
John Lovett explains that you can do a decent job of mixing red, yellow and blue dyes, but only if you allow yourself to use multiple blues and multiple yellows, for example. If you want to do the best job possible using only three subtractive primaries, then the best you can do is to use cyan, magenta and yellow.
So finally you can see that the best subtractive primaries are cyan, magenta and yellow because the cyan is red absorbing, the magenta is green absorbing and the yellow is blue absorbing. And what is more, you now understand why this is the case (rather than accepting dogma). You also understand why there is a relationship between the CMY of subtractive mixing and the RGB of additive mixing.
The optimal additive primaries are red, green and blue (I will cover this elsewhere). And for this reason the optimal subtractive primaries are cyan (red absorbing), magenta (green absorbing) and yellow (blue absorbing).
But don’t be fooled by this lovely subtractive colour mixing diagram. You might not get such lovely blue, green and red colours when you mix real CMY primaries (either on your printer or with inks/paints). Why not? Because of the unwanted absorptions.
If you want to to know more you could do worse that get a copy of Measuring Colour, now in it’s 4th edition, and authored by Hunt and Pointer.
This post gets quite a few hits so I will take this opportunity to direct you to my short series of youtube clips that describe the issues discussed in this post in a visual way. You can see them here. If you want something a bit more technical check out this short lecture on colour primaries or visit my patreon.
