An introduction to Colour Theory

The Colour Wheel

Throughout history, attempts have been made to ascertain how colours are created, and what happens when various colours are mixed. The Ancient Greeks developed theories as far back as 350 BC to describe how to create various colours, which were largely based on the fact that colours are only visible in light that is brighter than black or darker than the sun. From this they surmised that colours must be dependent on the amount of darkness or lightness present.

In 1704 Sir Isaac Newton challenged these beliefs when he published Opticks in which he demonstrated his “colour circle” and proposed that colour is actually a fundamental property of light itself, rather than a physical property of objects emitting colour. Newton’s colour circle was formed by taking the colours of the visible spectrum and wrapping them into a circle so that the red end joined the blue end. Using this device he was able to demonstrate a number of relationships between the colours we see, including the fact that mixing a small number of colours could result in white light.

Since Newton’s time, colour theorists have revised and developed colour wheels into the endless variety and complexity that can be found today. Generally colour wheels take two forms, those that describe the results obtained when mixing colours in the form of dyes or paints, and those, like Newton’s, which describe the effects of mixing light.

The RGB Colour Wheel shown here contains 12 colours, arranged in a circle where each colour is created by mixing the two adjacent to it. The colours Red Green and Blue are used by electronic devices such as computer screens, televisions, and of course cameras to create all the myriad colours required in full colour imagery. How this is achieved will be covered later, but for now, it is worth noting that these three colours are fundamental building blocks used in the creation of colour. By analysing the positions of all the colours on the colour wheel, it is possible to learn a great deal about colours and their relationships to each other.

The “Artists” Colour Wheel is based around three fundamental or primary colours – Red, Yellow and Blue which are considered as such because they cannot be made by mixing other dyes, paints or pigments. This is the most common kind of colour wheel you are likely to encounter. For our purposes, in the modern age of digital colour, we will concentrate on the “RGB” colour wheel which uses Red, Green and Blue as its basic colours from which all others are created.

The three key colours on the RGB colour wheel are of course Red, Green and Blue. These are the building blocks we use to create all the other colours on the wheel. These colours are fundamental and are known as “Primary Colours”.

Positioned half way between each pair of Primary Colours are the Secondary Colours, Yellow, Cyan and Magenta. These colours are created by mixing equal amounts of the two Primaries closest to them. For example Cyan is created by mixing an equal amount of Blue and Green. It is also worth noting that each Secondary Colour does not contain any of the colour opposite it on the Colour Wheel. So Yellow contains no Blue, only Red and Green, and for this reason it is sometimes known as “minus blue”.

To complete the colour wheel we insert colours in the positions between the
Primary and Secondary colours.

These colours are once again created by mixing those colours adjacent to them so for example between red and yellow is orange which is created by mixing red and yellow equally. Although we have a name for the mixture of red and yellow (orange) some of the other tertiary colours do not have their own names. Consequently it is more normal to name the Tertiary colours by the colours that are used to create them. Hence we have Red-Yellow, Green-Yellow, Green-Cyan, Blue-Cyan, Blue-Magenta and Red-Magenta (where all the names take the form Primary-Secondary).

Hue, Saturation & Value (HSV)

At this point it is worth noting that all the colours we have been considering here are pure colours in that they contain no white or black. The term used to describe such colours is “Hue”, which can be considered as the dominant wavelength in any light source. Red, Blue, Green and Yellow are examples of Hues, whereas pink and scarlet both have the Hue Red.

Imagine tipping the colour wheel on its side and then extending it to three dimensions, so that it forms a cylindrical shape. In the centre of the wheel on the top surface of the cylinder is white and at the centre of the bottom surface is black. Throughout the depth of the cylinder at the centre of the circles are all the infinite tones ranging from white to black.

When colours are mixed with white (or made lighter) they are no longer of pure Hue, and are known as “Tints”. When mixed with black (or made darker) they are known as “Shades”. The amount of white, black or grey that is mixed with any Hue defines its “Saturation” and colours near the central axis of the cylinder are “de-saturated”. The lightness or darkness of any colour is dependent on the tonality of white black or grey mixed with it. This tonality is known as the “Value”

By using the three values of Hue, Saturation and Value it is possible to precisely define any possible colour.

Complementary Colours

Now that we have defined the colour wheel, we can start to look at relationships between colours, how they interact with each other and various properties of specific sections of the colour wheel.

Any two colours opposite each other on the colour wheel are known as “Complementary Colours”. In black and white photography, the key compositional components are tone, texture and form. In colour photography we add to this list colour, and complementary colours provide the greatest “Colour Contrast”. By limiting the colours used in any composition to two complementary colours, we maximise the colour contrast, which gives greater impact to our images. It doesn’t matter which two complementary colours we choose, blue and yellow, green and magenta or red and cyan, any combination of two colours on opposite sides of the colour wheel will make the colours used appear more vibrant, particularly if they are of pure Hue.

Colour Schemes

Any three colours equally spaced around the colour wheel are known as “Triadic” colours. Any group of triadic colours offer us the maximum colour contrast that can be obtained from a composition limited to just three colours.

The way to maximise colour contrast using four colours is to use “Tetradic” colours, or two sets of Complementary Colours.

In a Split Complementary colour scheme, a single colour is selected to dominate, then instead of contrasting it with its direct complement, we choose the colours that are analogous to that complement. In this scheme, colour contrast is high, but the two analogous colours also introduce a level of harmony.

Colour Harmony

The simplest way to maximise Colour Harmony and minimise Colour Contrast is to utilise colours of a single Hue. By using just tints and shades of one colour we create a “Monochromatic” colour scheme and effectively our compositional tools are restricted to those used in black and white photography, tone, texture and form.

An alternative to the monochromatic approach is to use colours that are next to each other on the Colour Wheel. Any group of colours that are next to each other are said to be “Analogous”. Each colour on the colour wheel is created by mixing the two colours adjacent to it, and as such the “Colour Contrast” between analogous colours is at a minimum. In analogous colour schemes, it is usual for one colour to dominate and for the surrounding colours to provide highlights and enrich the overall appearance.

When looking at images you regard as successful, try and work out which of the colour schemes mentioned here has been used and why it might contribute to the success of the image. By practicing in this way, you will find that compositions with high contrast or overall harmony will become a natural part of your own work.

Additive Colour

The modern understanding of the properties of light and colour owes a great deal to the pioneering work carried out by Sir Isaac Newton at the latter end of the seventeenth century. Newton showed that a prism could break white light down into its component colours, and that a further arrangement of prisms could refract the light back in to its original form. In his publication OPTIKS he famously stated “For the rays to speak properly are not coloured”, his way of saying that colour as a property is a psychological phenomenon and not a physical one.

An Additive Colour System relates to light that is emitted. It does not refer to an object that has the property red – only to a light source that is emitting red light. An additive system requires three light sources, each emitting one of the primary colours red, green and blue. If we mix two of these light sources in equal amounts, the resulting colour is one of the secondary colours cyan, yellow and magenta. When mixing equal amounts of all three light sources the result is white light.

The additive colour system using coloured light sources, confirms the facts we discovered in the section on the colour wheel. Yellow is made from Red and Green and contains no Blue. It should also be clear that a mixture of red and green light in equal amounts (light of two very different wavelengths) is physically different from pure yellow light (with a single wavelength of roughly 580nm). The important thing to realise is that both stimulate our eyes and brains to give the same result that is a psychological impression that we call “yellow”.

The brilliant Scottish physicist James Clerk Maxwell is considered to be the founder of additive colour synthesis. In 1861 Maxwell presented to the Royal Society in London a demonstration based on ideas he had introduced at the Royal Society of Edinburgh some 6 years earlier. He had arranged for the photographer Thomas Sutton to photograph a tartan ribbon 3 times (using the available black and white film), each time with a different filter over the lens – Red, Green and Blue. The three images were then projected onto the same screen and each projector lens was covered with the same colour filter used to take the original image. When the three projections were brought in to register a full colour image was seen to appear.

As a footnote to Maxwell’s demonstration, we now know that he was extremely lucky that it worked at all! The black and white film used by Sutton was only sensitive to light from the blue end of the spectrum and shouldn’t have recorded anything through the red and green filters. An attempt to repeat the experiment nearly 100 years later showed that Maxwell’s green filter had in fact allowed some blue light through, and that the ribbon’s red colours were reflecting ultra violet which was recorded with the red filter in place.

Subtractive Colour

Subtractive colour theory explains how to mix inks, dyes, paints and natural colourants to create colours which absorb some wavelengths of light and reflect others. An object that absorbs one of the three primary colours from the additive world (perhaps Red) will reflect the other two (Green and Blue) and appear to be the colour of the reflected light (in this case Cyan). It follows from this that an object that absorbs Cyan is effectively absorbing Green and Blue and will therefore reflect only Red. The colour of any object viewed under white light is determined by the wavelengths of light its surface reflects and absorbs.

When viewed under a white light, a nice ripe banana appears Yellow. This isn’t because it emits Yellow light (the additive world) but because it absorbs Blue light and reflects only yellow. Without the white light to illuminate the banana, it would be in the dark and would appear to have no colour at all.

The primary colours of the subtractive world are Cyan, Magenta and Yellow, so it should come as no surprise that the secondary colours are now Red, Green and Blue.

Anyone who owns an inkjet printer will be familiar with the subtractive colours as these are the ink colours used in printing processes. Cyan ink for example absorbs Green and Blue light and can therefore be used to control how much red is reflected. By altering the amount of each ink that is placed on the paper it is theoretically possible to produce almost any colour. In practice, however, impurities in the manufacture of inks mean that it is impossible to create true blacks.

When mixing Cyan, Magenta and Yellow, all that can realistically be achieved is a muddy brown. For this reason a fourth colour ink Black is added to the process. The CMY inks are used to create the Hue and the Black (K) ink is used to control the Value. The subtractive printing process is often referred to as CMYK after the four colour inks used in printing.