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An Introduction to Colour Theory

I find colour theory fascinating. As a designer it helps practically to know the theory behind colour and what terms like RGB, CMYK, etc mean. Often you will be asked to prepare a file in a CMYK format for example. And as someone with a background in Astrophysics (I used optical spectroscopy during my PhD) I like to delve deeper into the science behind colour theory.

Have you ever wondered why printers use CMYK (cyan, magenta, yellow, black) inks, while monitor screens are made up of RGB (red, green, blue) pixels? But, you were taught at school that the primary colours are RYB (red, yellow, blue) right?

It's all down to the way our eyes perceive colour.

Inside our eyes we have photoreceptors called cones and there are (usually*) three types.

Each type of cone is sensitive to a different band of light: red, green or blue. If we see red light, our red cones are stimulated and this subsequently tells our brain, that is the colour red. The same happens for green or blue light.

*When one or more cone types is missing or faulty then varying degrees of colour-blindness occurs - where affected individuals cannot distinguish as many colours.

But what about the other colours? Well, if we see yellow light, for example, it stimulates both the red and green cones in lower amounts and our brain then interprets that as yellow. By stimulating the three types of cones in varying amounts, our brain then can detect millions of different colour hues.

A consequence of this clever set up is that, for example, our brain cannot distinguish between pure yellow light, and a combination of red and green light in the right quantities.

So when we need to light up a dark screen with coloured light - televisions, computer monitors, phone screens etc. - we only need to use the three colours corresponding to the cones in our eyes: red, green and blue. Our brains can be tricked into seeing a large range of colours from just those three key ones at varying intensities.

This is why RGB colour codes come in sets of three, one number between 0 and 255 representing the intensity of each of the colours red, green and blue.

(Hex colour codes are just six digit translated versions of RGB codes that are easier to use when coding websites etc.)

If you are a designer who works with digital artwork - art that will be viewed on a lit screen - then you will need to design in RGB colour.

When we are dealing with coloured light such as being emitted on a screen, this is called an additive colour system. No light being emitted = black. All colours being emitted = white. The more colour is added the lighter the colour becomes.

This is in contrast to the CMYK system, which is a pigment-based system rather than a light-based system. Most modern printers use the CMYK colour system for printing inks onto paper, card, fabric, etc. As adding an ink colour absorbs/subtracts from the total colours reflected back, we call the CMYK system (or indeed any colour system created with dyes or paints) a subtractive system. No colours printed = white. All colours printed = black. The more colour is added the darker the colour becomes. The opposite to the RGB system.

This is because, unlike the pixels making up a monitor screen, the printer ink itself does not emit light. What happens is ambient light hits the paper and a proportion is reflected back at us.

If we start with a plain white piece of paper (an absence of all ink colours) then all light colours are reflected back to us. The red, green, and blue cones in our eyes are all stimulated and our brain sees white.

If we add a coloured ink to the paper, e.g. yellow, then what actually happens is that all the colours except yellow are absorbed, and the other colours are reflected back. Both the red and green cones are stimulated in our eyes.

Each of the colours in the CMYK system (cyan, magenta and yellow) are specifically chosen to stimulate two different cones in our eyes at a time. They are, in effect, the opposites of RGB, and this is the most efficient way to produce the largest range of colours.

(The 'K' stands for black - because in practice, if you mix CMY you get a muddy grey, so in modern printers a separate black ink is used as well.)

CMYK colours will not be as bright as RGB colours - they are not backlit like the colours on the screen and are derived from natural and synthetic pigments rather than emitted light. For this reason it is often recommended that if you are creating a design for printing on fabric or paper you design in CMYK from the start rather than convert at the end. Essentially this means the program you are using to design with will limit your colour palette to ensure a more realistic representation of final colours.

CMYK is not the only colour system used in the printing industry - it just tends to be the most efficient in terms of colour range versus affordability. Greater colour ranges and precise colours can be achieved by combing more than four inks.

For example, it's impossible to create neon or metallic colours using just CMYK inks, more specialised pigments are needed.

The Pantone Matching System, one of the most famous printing systems in use, has 13 base colours plus black used to produce its large range.

Spoonflower, the fabric printing on demand company, uses up to eight base inks in its printing. For this reason Spoonflower actually recommends RGB rather than CMYK to be used when designing for the most realistic colours.

So, finally, why do we use cyan, magenta and yellow in most printers instead of the primary colours red, yellow and blue?

At school you may remember being taught that the primary colours are red, yellow and blue, and that all other colours can be mixed using just these three.

Well this turns out to be not quite correct. Have you ever tried it? It's impossible to get bright purples, magentas, lime greens, turquoises etc. from a combination of red, yellow and blue.

Before the invention and discovery of modern pigments, it was considered that red, yellow and blue were the primary colours simply because magenta and cyan pigments were just not available. Artists would not have these colours to paint with, so red and blue were the closest approximations. And it is true that you can produce a wide range of colours with RYB.

Since the invention of modern printers and pigments, it has been possible to produce vibrant cyans, magentas and yellows and hence produce a much wider range (called a 'gamut') of colours.

If you compare the CMY and RYB colour wheels below, you can see the difference in the hues - RGB produces a much earthier, muddier set of colours.

Obviously, there is nothing stopping you choosing the RYB palette deliberately so you can replicate that 'earthyness' in your artwork if you like it! But I think it's helpful to be aware of the two different colour systems to help you achieve the colour palette you want, and in case you are wondering why your rainbows aren't quite as vibrant as you want them to be.

So why do still teach RYB? Tradition, mostly, also it makes it simple to teach to children as it contains the colours of the rainbow (red, orange, yellow, green, blue, indigo, violet) rather than any additional colours.

And why are there three primary colours? Why not four? That basically comes down to those cones in your eye - three cone types = three primary colours. It's called trichtomatic vision.

So if you are thinking about getting your first set of paints it can tempting to buy all the pretty colours, but really, you can get by with just three.

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