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The Colours of the Rainbow


Little did I know when I started the annual discussion with my (art degree) wife about colours that I would start a whole feud off about the subject. This blog entry is a TAD technical but hopefully not enough to put you off – if you think the 3 basic (primary) colours are red, yellow and blue then you really DO owe it to yourself to spend a moment to read on… with an open mind you’re about to learn something new and interesting…

So the argument goes that red, yellow and blue are indivisible – they’re the colours that can’t be made. So how is it that every printer on the planet makes red from magenta and yellow?  Caught your attention? Read and learn…

The basis of the argument is this… artistic people are (were – not checked with any youngsters) taught that red, yellow and blue are the “primary” colours – that is the very basics of colour in some folk’s eyes.  I realised this when Maureen put up a “poll” on Facebook, which produced several responses indicating that many believed red, blue and yellow to be the primary colours. There were a few other responses which were just plain wrong, but this one deserves some attention.

Well, Facebook is REALLY not the place to discuss this because it is primarily oriented to quick information bites… and really, developing any logical argument is almost impossible.

So here is my attempt, no doubt flawed, to set the record straight.

Red, yellow and Blue are NOT, I repeat NOT any kind of “primary” or indivisible colours. They form what is called a (not THE) set of “subtractive” primaries – indicating that if you are mixing paints – then these COULD be considered to be the only three colours you need but if you are walking around with a paintbrush and those three colours-  you might want to read this carefully – as you are missing out.

The problem with this is that firstly it is very old school and secondly the logic is just plain wrong – suggesting that these are the only colours that are “indivisible”.

So – the argument goes that red, yellow and blue make any colour and cannot themselves be made.

Paint us just one side of the story, magenta, cyan, yellowbut before going off on a tangent – let’s stick with paint for a minute. You know that printer sitting next to you? The one that DOESN’T use red, green and blue?

Yes, that one – it uses magenta, cyan and yellow – and though it almost definitely DOES include black, it is actually possible to make a half-arsed black from these colours – more than can be said for red, yellow and blue.

Yes, you CAN mix magenta and yellow to get the supposedly primary RED! Yes you can mix cyan and yellow to make the supposedly primary green etc… you can make blue from magenta + cyan and yes you CAN do all of this on paper. Printers do it all the time.

But this is not where the story ends – not by any means – in fact is that what some call primaries (in the simplest sense) depends on what you are doing – if you are ADDING colours- i.e. one spotlight adding to another or on your TV or computer monitor – then the three colours you should consider “primary” are red, green and blue.  Things there are very different to when you mix paints and “subtract” colours. For example, mixing the three paints above produces a kind of black. Mixing the three additive (light) colours – gives you WHITE!

Ask any television – the ONLY colours that any modern television has access to are red, green and blue ( no, REALLY – get up there and take a close look at your TV or any kind of colour monitor – there are ONLY three colours, no white, no yellow etc) – and from those three colours, the TV can present just about any shade of white, any colour and black, grey, silver, gold, – you name it.. i.e. a “complete” set of colours. Many TV manufacturers claim a range of millions of colours – all from nothing more than red, green and blue. You do of course realise that gold and silver are not colours as such.. but I digress.

But this is all part of a discussion differentiating ADDITIVE (typically light beams) colours and SUBTRACTIVE (typically paint) colours –  let’s get back to basics – the EYE.

The EYE is the main arbiter of colour (though it is ultimately the brain that decides what to do with that information) – if it were not for our eyes who would be to say what GREEN is – and even then that is only an agreed standard between us – if we ALL saw the SAME colours in the SAME way then there would be no such diversity of “favourite” colours! There are indeed apparently, women with extra sensors who can see colours the rest of us can’t imagine. All this means of course is that they can detect a slightly wider range of “colours”.  If you’re not aware, visible light is merely a narrow part of a spectrum that goes WAY above and below commonly sensed colours.

The eye, the very core of our visual sense, has two mechanisms for detecting light –the RODS – which detect brilliance (intensity) and the CONES which detect colour (frequency).  There are THREE kinds of pigmented cone and their main areas of sensitivity hover around red, green and blue.  Shine a narrow frequency green light into the eye and only one of the three cone types will respond – etc.  You don’t get any more “natural” than this when it comes to what are the main colours.

How the brain handles that is a WAY more complex issue – but for those who think it is somehow magical and so not relevant, consider this – we DO now understand this to the level that computers have been able to “guess” what someone is seeing by looking at the brain – but that discussion is for somewhere else as what we actually see is somewhat different to what we might intuitively think we see.

If someone shines red light and green light on a piece of paper, our eyes pick up both of those bands of reflected radiation and agree this is “YELLOW” though in fact, yellow is indeed a frequency range of it’s own whereas red and green light falling on paper is a MIX called yellow – like two different tones – unlike sound-  where we would hear those individual tones – our eyes decide that this is in fact one colour. All of this information is freely available on the web – just avoid anything that mentions the word “alien”. Yet we treat this false yellow as almost the same as “real” yellow. See the yellows on this screen. They are NOT yellow – green and red lights are firing at your eyes. If we see true yellow-  then again both the cones responsible for the green (middle) colour range and the cones responsible for the red (long) colour range pick up on that.

Like a camera – we see intensity and we  see colour. But there the similarity ends.  Take a piece of white paper with a black dot and stare at it for a while… shut your eyes. What do you see –  a black rectangle with a white dot in it?  Look at a negative image for a while – stare at it – close your eyes, do you see a full-colour positive image? Yes.

This can be explained relatively easily.

Take a camera and point it to the same white sheet with a black dot. Take as many pictures as you like and they will always be the same – the white sheet with the black dot. Sounds good – but then – take a sunset and see if you can get the details in the grass and buildings while simultaneously seeing the full sunset – you CANNOT achieve the same result as your eyes with a camera without using a PLEASANT but unreal technique called HDR.

Eyes have something cameras do not – pixel-level desensitisation.  That is, the smallest point of light you can see – individual sensor cells – can handle massive ranges of light and dark that cameras generally can’t handle and this goes WAY beyond a simple iris which can be made larger and smaller to make the eye more or less sensitive to light.   As well as that, the actual individual sensors can “de-sensitise” or become more sensitive as needed. This helps us handle visually a wide range of intensity – but of course also ensures we see something that is not actually anywhere near reality.

Hence we we can (kind of) see all the detail in the horizon from the sunset to the landscape below… NOT just because of the iris in our eyes but because parts of our eyes can re-organise sensitivity as required.

If you accept this – then everything else falls into place and also explains why our eyes are prone to so many optical illusions which cameras are not (there are other brain-related reasons here but that is beyond the scope of this article, suffice it to say that the brain is able to further process vision to “see” patterns that might not be there.

From a survival perspective, the ability to pick out potential predators from “noise” is very important – but now that we live in cities we are cursed with an eye-brain interface that, rather than give up on recognising patterns when there are none – will do it’s DAMNDEST to find order in the disorder even when no such order exists in reality. There are many “trick” images out there which prove this point. Ever looked in the dark and THOUGHT you saw something? Ever listened to white noise (the sea or general random noise) and THOUGHT you heard something recognisable – we all experience this at one time or another… but I digress.

Take three torches of the right brilliance – one red, one green, one blue. From a distance if one shines all three at you – you WILL see WHITE.  Again check out your TV – it absolutely does NOT produce white natively– take a close look – you will see tiny red, green and blue elements – move back – they look odd – look back further still and they look WHITE. Red, green and blue elements can present you with just about any colour you can imagine – just by varying proportions. Turn them all off and you have black – unless your screen reflects your surroundings.

Why would I know about this stuff?

As well as taking a keen interest in painting at school, if you are familiar with RGB LED lighting, this comprises red, green and blue LEDS – by varying intensity you can not only produce pretty much any colour but also any shade of white from COLD to really WARM. I spent months getting to grips with this and the various colour models (RGB, CMYK etc) Some technical folk use RGB as the base for producing colour but possibly more intuitive are other models which allow you to pick a colour – but then blend it from 100% saturated at one end to white at the other end.  With more and more people using coloured lighting at home, this is important.


Look at the image above – look at that yellow – that’s a nice yellow isn’t it. NO IT IS NOT – it is green and red – take a CLOSE look at your screen. Same with the CYAN colour. The colour ranges are merely frequencies of electromagnetic radiation with blue at the shorter (higher frequency) end and red at the slightly slower end… enough of being technical – it is all out there if you are interested. In that image above – the only TRUE colours are red, green and blue.

That is the reality – somewhat more complex than the simple 3-paint-colours-makes-anything that seems to be the takeaway from art school. But all the richer for it… everything I’ve discussed here can be EASILY tested without ANY special equipment or cost – just an open mind. Looking at the bigger picture often helps gain a better understanding of the detail within.

Now back to aliens… colours are one thing… what the brain DOES with that information is another. Your brain is NOT at the end of a wire with the neat, simple colour image being fired at it.  Far more important to the animal brain (sometimes) is movement – so what the brain actually gets is a series of packages and one of them is totally devoted to MOVEMENT, another colour etc.… so depending on what is most important at the time, you can ponder the beauty of subtle colours – or you can get the hell out there because you’re about to be run over by a car.

There was a time when this was all mystical but as I mentioned above, today we are on the edge of a future where computers will read the messages the brain processes – and extract images from that – depending on your viewpoint that is either a terrifying future – or a beautiful vindication of science – personally I think it is somewhere in the middle but I am dying to see where we go next. What to me is more important is that we are now able to have a fairly comprehensive view of the properties of light and how humans and some animals process it.

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