Understanding Colour Models

When developing artwork for your website or design project or when brokering your print job, you will often hear us refer to different colour models; specifically, RGB, CMYK and spot or PMS colours.  To appreciate the differences between these models and how they play a role in the look and feel of your brand and subsequent advertising pieces, some basic colour theory and understanding is needed.

Colour is a visual perceptual property derived from the spectrum of light based on varying wavelengths of light energy – the visual gamut. The colour your eye sees is a sum of the reflected light from an object interacting in the eye’s special light receptors and interpreted by your brain.  Light contains both a visual spectrum and an invisible spectrum (as anyone with a TV remote can attest to the infrared friend). The visual spectrum is seen as colour and is beautifully represented in nature by a rainbow which contains all the colours; red, orange, yellow, green, blue and violet that can be produced by visible light. The invisible spectrum sandwiches the visible spectrum with that remote’s infrared on one end and ultraviolet (UV); neither of which can be seen by the human eye but certainly can be felt as a sunburn or witnessed in the flipping of your favourite channels.gamut

Within our visible spectrum there are boundaries called colour spaces.  The diagram at right shows the entire visible spectrum with the property lines of these spaces as an overlay which illustrates the limits of each. As both web (light) and graphic (print) designers, we must work in both these spheres and also with their limitations and even inabilities to cross-over certain colours between one another.  Let’s take a look at each colour space separately.

RGB (The Additive Colour Model)

Additive Colour Model (RGB)

Comprised of Red, Green and Blue light as the primary colours or RGB for short.  Most anyone who is a TV or computer user on a regular basis will certainly have at least run across the term RGB  in plugging and unplugging cables or doing any basic video troubleshooting. RGB is the colour space of the digital age and any display screen including monitors, HD screens, cameras and scanners because they use light to recreate or simulate colour.  Before the electronic era however, RGB was already a sound model as it most closely represents the human eye and how it perceives colour.

This is called an “Additive Colour Model “because the three primaries add-up to make white light. The degree or quality of white light generated depends on the chromaticities of the input colours as RGB doesn’t specifically define them – if they are defined the colour model becomes an absolute colour space and we find ourselves deep into colour theory!  RGB is the largest space within the visual gamut with all other colours being blends of these three plus their secondaries.  Until recent technological advances, the digital RGB palette of 16.7 million colours posed a problem for many computer and Internet users as their devices or connections were not powerful enough to show all the colours and blends. This has largely been overcome now for most users as limits in colour depth of screens is no longer an issue like when the web first got started and most monitors showed a maximum of 216 colours.

Notice that the secondary colours of the additive model are the primaries of its twin, the subtractive or CMY colour model.

CMY (The Subtractive Colour Model)

By comparison to additive, the subtractive model is far more limited in its gamut and is created with pigments, paints and dyes and relies on the reflection and wavelengths of available light to product colour. For example, plain paper absorbs all colours, making it appear white. Placing ink or toner on that white paper acts as a filter causing certain wavelengths to be reflected which your eye then sees as colour. This is called a subtractive colour model as you are filtering or subtracting light away from the pure whiteness of mixed light and blank paper.  If the light is not white (such as a dim light bulb or overcast day), although our eye is very adaptive to compensating the true colour may not be perceived correctly. As you can see subtractive colour is completely opposite to additive which starts with darkness and adds wavelengths of colour to achieve white, here we start with white and add pigment to achieve colour ultimately ending in a muddy black (composite black) if all the light from the reflecting source is filtered out.

The primary colours of Cyan, Magenta and Yellow are the secondary colours of the additive model just like RGB is the secondary colour set of this model.  Thus you will often hear a printer refer to this as the CMY colour space.  Again, combining or filtering all light out in a subtractive model results in darkness or composite black.  However, composite black is not pure black and quite often in print and paint ends up a deep muddy brown and thus in commercial paint and printing a pure or artificial black (key black) is added to enhance the model.  This is where the K in CMYK comes from.  combine to form composite black.  Notice that the primary colours of a subtractive model are the secondary colours of the additive model above, and the secondary colours of the subtractive model are the primary colours of additive. The “K” in CMYK comes from the addition of true black as composite black is really a mud brown. The CMY colour space is far narrower because of its subtractive nature.

“Spot” Colours

These are colours generated and formulated specifically into printing inks used primarily in offset printing methods. They may include the addition of pure or mixed colours like orange or green to main process (CMY) to create CMYKOG but more often refer to proprietary colour systems like Pantone, DIC, and TOYO to name a few.  The PMS (Pantone Matching System) if generally the most know of these, especially with the expansion of PMS colours out of the press room and into mainstream home fashion.  Spot colour is generally used to obtain a specific hue or value  that is either out-of-gamut of CMYK printing or which may even be trademarked to a brand such as PMS 165 Orange to Harley Davidson Motorcycles.  Spot colours can also include metallic properties, fluorescent reflectivity, varnishes or be run with double imprint for additional intensity and coverage and deliver consistent results every time when colour match matters.

Today’s lower priced digital printing delivers process (CMYK) colour at a fraction of the price a decade ago, making spot colour a premium print solution and costing hundreds of dollars more on a run.  This has many brands and designers rethinking their brand colour strategy to a “close-enough” compromise to the once powerful statement a spot colour would make on marketing pieces.  It seems gone (or significantly reduced) are the days and artistic license of vibrant and intense colour only delivered by a custom-mixed ink like Cobalt Blue which actually contains cobalt salts of alumina and simply can’t be recreated mixing the subtractive primaries suitably. Using Spot colours in your project help deliver consistency, far more vibrant colour, specific hues and special effects, but now at a higher printing cost.