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Colour Theory

by Andy Slater

Primary colours? Secondary colours? Complimentary colours? What the heck is it all about and why should we care anyway? The reason that we, as model makers, should care is because an understanding of it can help us to use colours more effectively. For example, when we need to mix paint. What I am going to attempt in this document therefore, is to explain some aspects of colour theory that are useful to model makers. At the same time I will explain some of the other stuff which often causes confusion.

Primary Colours

There is a lot of confusion about primary colours and for good reason. As a child I was told that they were red, yellow and blue and that I could mix these colours of paints to make any other colour. It works pretty well too. Later however, in science lessons, I was told that the primary colours were red, green and blue and indeed I observed that television and computer screens use red, green and blue dots to make up their pictures. More recently I noticed that my inkjet printer cartridge contained cyan, magenta and yellow inks and my friend who is a printer tells me that these are the primary colours. So what on earth ARE the primary colours? The fact is that there is more than one set of 'primary colours' as we shall see:

Additive Primaries

The additive primaries are red, green, and blue and are used when mixing light or adding colours of light together. The reason that the primary colour phenomenon exists at all is because the retina of the human eye contains receptors that are 'tuned' to different colours of the spectrum, namely to red, green and blue light. Thus we see colours as a combination of these components. On a TV or computer screen (which is generating light) quantities of red, green and blue light are combined to create the various colours that we see. In computing parlance you'll often see colours described using their RGB (Red, Green, Blue) values.

When the additive primaries are mixed in equal intensities we get the following colours:

Green + Blue = Cyan

Red + Blue = Magenta

Red + Green = Yellow

Red + Green + Blue = White

Subtractive Primaries

When we look at a TV or computer screen we see light being EMITTED from it. Most objects however do not create their own light and what we see is light being REFLECTED from them. The reason that a piece of white paper looks white is because it is reflecting white light (which contains all colours). If you look at the white paper under coloured light (which only contains some colours) then it will look the same colour as the light because it is only able to reflect whatever colours of light are falling on it. Pigments (inks, paints, etc) applied to our white paper absorb colours from the light falling on them and reflect what's left. The pigments SUBTRACT colours from what we see.

Another way in which colours can be subtracted (and which is perhaps easier to understand) is by putting coloured filters in front of a light source. As we see from the section above on additive primaries, cyan light is made up of green and blue light. If we put a cyan filter in front of a white light source (which contains all colours) we see cyan because the filter is subtracting the red light and allowing only the green and blue light to pass through it. If we were to then put a yellow filter in front of the cyan filter it would subtract the blue light and we'd be left with green light.

Hopefully because of this you will understand that cyan, magenta and yellow form a set of 'subtractive primaries'. This particular set of subtractive primaries are frequently used by printers (see Printing Primaries below) but it should be noted that these are by no means the only set of subtractive primaries. Another set are describe below in the section on Painting Primaries.

So why are there more than one set of subtractive primaries?

Firstly you have to appreciate that the only reason that there are three colours in the set of additive primaries described above is because there are three types of colour receptors in our eyes. There is nothing to say that subtractive primaries should come in groups of three.

Secondly, you should need to be aware that although it is theoretically possible to mix any colour from the set of subtractive primaries described above (cyan, magenta and yellow) the fact is that these would have to be precise shades for it to work and alas, when we come to the real world (and we have to make our pigments from real substances) we can't just go dig up pure cyan, magenta or yellow. The pigments that we make are imperfect because they are mixed up from other coloured substances that we can 'dig up'. This is the reason that a) there is more than one set of subtractive primaries b) not all sets are limited to just three colours and c) black is nearly always added on as an 'extra' although it isn't usually classed as being one of the primaries itself.

Painting Primaries

These, as most of us were taught as children, are red, blue, and yellow. They are used when mixing pigments and therefore form another set of 'subtractive primaries' (Note that they are not the only set of subtractive primaries - see Subtractive Primaries above). When mixed in equal quantities the painting primaries give the following results:

Red + Blue = Purple

Red + Yellow = Orange

Yellow + Blue = Green

Red + Blue + Yellow = Black

It's important to note that you can't just mix any old shade of the colours and that achieving a good black in this way is so difficult as to be not worth trying. It is fair to say however that if you want to do some painting on a very low budget, that buying pots of red, blue, yellow, black and white will allow you to mix up most of the other colours that you will require.

Printing Primaries

As explained in the section on Subtractive Primaries: cyan, magenta and yellow are a set of subtractive primaries and are the set most commonly used by printers. Thus they are commonly referred to as the 'printing primaries'. As with the painting primaries described above, it is difficult to achieve a good black from just these pigments so printers will usually also use black. Printers sometimes refer to colours in terms of their CMYK (Cyan, Magenta, Yellow, blacK) values.

The reason that printers use this set of primaries as opposed to the painting primaries described above is that they yield superior results. If you are painting however, you will find not only that it is much harder to buy the appropriate shades of cyan, magenta and yellow paint, but also that mixing them is far less intuitive than when mixing the painting primaries.

Fundamental Primaries

See 'Cone Primaries' below.

Cone Primaries

As explained above in the section about Additive Primaries, the receptors (called cones) on the retina of the human eye are 'tuned' to respond to red, green and blue light. They do not however, respond exclusively to one particular wavelength. Instead they respond to a range of wavelengths to a greater or lesser extent. It is the case that while the red receptors respond best to red light (of various wavelengths), the other receptors (green and blue) will also respond to some wavelengths of 'red' light, but to a much lesser extent. The brain is then responsible for sorting out all of this information and determining what we actually see.

The cone primaries are theoretical colours that the brain would perceive if each of the three cone types could be stimulated individually (which is impossible) and are described as 'extreme red', 'psychedelic aquamarine' and 'extreme purple' in a most interesting document on the University of Colorado website which you can find here

Psychological Primaries

Used when discussing the psychological or perceived effects of colour, these are red, green, blue and yellow (black and white are often also included).

Secondary Colours

Secondary colours are colours which are created by mixing two (but not three) primary colours. The resultant colour will of course depend on which set of primary colours you are using and in what proportions you mix them. (See also: Tertiary Colours).

Tertiary Colours

Tertiary colours are created when you mix a secondary colour with some of the primary colour that was NOT included in it. Or, to put it another way, a tertiary colour is one that contains elements of ALL THREE primaries (from whichever set ot primaries you are using).

Complementary Colours

Colour TheoryThe 'colour wheel' to the right show shows the additive primaries (red, green and blue) and the secondary colours that can be made from them. Note that tertiary colours are not shown. Note also that the secondaries created by equal quantities of the primaries (cyan, magenta and yellow) are also indicated.

Using cyan as an example, we know from what we have said earlier that pure cyan is made up of green and blue light with no red. For this reason cyan appears opposite red on the colour wheel. Cyan is effectively 'as far away from' red as you can get or to put it another way, red is everything that cyan isn't (and vice versa). Red and cyan are therefore referred to as being complementary colours. Red is the complement of cyan and cyan is the complement of red. Similarly the complement of any secondary colour can be found opposite it on a colour wheel.

This is useful to know for a couple of reasons:

Firstly, from a design point of view, putting complementary colours together will make both appear more vibrant because of the contrast between them.

Secondly, if you want to darken a pigment, it is generally better to mix in some of it's complement rather than using black (which can end up looking 'muddy'). Remember however that the colour wheel to the left is for additive colours (I've used it here because it's being displayed on a screen) and if you are mixing pigments you should use a colour wheel with the appropriate set of subtractive primaries.


Hopefully this document has improved your understanding of primary colours. If not, then probably the most useful things to be aware of (from a model making point of view) is the information in the sections about 'Painting Primaries' and 'Complimentary Colours' while most of the rest can be regarded as worth being aware of but not worth worrying too much about.

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