Colour is an area of printing that is often misunderstood. Many are unsure as to the differences between the RGB & CMYK colour spaces and the differences between Spot or Process inks. This colour guide has been put together help explain these differences.
RGB v CMYK
Spot v Process
RGB V CMYK
RGB stands for Red, Green, & Blue. Colour is a form of light energy that comes in waves. The visual spectrum is continuous, but for convenience we divide it into red, orange, yellow, green, blue, indigo, and violet. The most dominant colours in the spectrum are red, green, and blue.
So when we talk about RGB colour, we are in fact referring to colour as we see it. Specifically, we are talking about light waves, such as the ones that come from your computer monitor. When you are designing for the Internet, RGB is the colour space that you use. Desktop colour printers (such as ink jet & colour laser printers,) are designed to interpret RGB colour, and translate it into ink on a page.
CMYK stands for Cyan, Magenta, Yellow, and Key (black). These refer to the ink colours used in the CMYK or "4 colour process" type of printing. Combining these colours of ink allows for reproduction of thousands of colours, and is sometimes called "full colour" printing. This can be deceptive, because there are colours that cannot be reproduced using the CMYK process. However, for most purposes, such as photographic reproduction, CMYK is completely sufficient.
So if you plan to send a colour job out to be printed, most likely you will need to supply CMYK art. This type of art can be separated by page layout programs such as Adobe InDesign and used to create film, which in turn is used to create the final plate that actually transfers the ink to the paper.
RGB = light. Use for Web & personal desktop printers.
CMYK = ink. Use for commercial printing projects.
Spot Colour vs. Process Colour
CMYK can produce thousands of colours, and is sufficient for most colour projects. But what if your project calls for a very specific colour, one that CMYK can't quite match?
It was for just such a purpose that the Pantone Matching System was created. It allows you to specify exactly what colour you want to use in an area of your project. Most commonly, it is used for logos. That way organisations such as Bankwest, The Commonwealth Bank and Telstra can achieve their corporate colours on every print job.
So, when you print using CMYK, you are using Process Colour. (The ink is laid down in a 4 step process, one ink colour at a time. Colours are built up using the combination of the inks, and the way they interact with each other.)
A Spot colour, on the other hand is a specific colour, usually a Pantone colour that is mixed to specifications and printed by running it through the press just the one time. Many publications are printed using two spot colours, as this can provide a pleasing addition of colour to a project without the expense of full process colour.
An example of the difference between a Spot and a Process Colour can be seen when printing the Pantone 185 red colour below:
In the example above, Pantone 185 Red is printed as a solid, by using Pantone 185 red ink. You can view all the Pantone solid colours in the Pantone formula guide set.
If we compare how the same colour looks as a spot colour vs. a process build colour, the image below has Pantone 185 red solid on the left, and the CMYK build (92% screen of Process Magenta, and 76% screen of Process Yellow) on the right.
As you can see, the colours do not match exactly. This is why you should always use a Pantone solid or spot colour when colour is critical. The CMYK built colours are not always a good reproduction of the Pantone solid colours.
Spot lamination and UV coatings are sometimes referred to as spot colours, as they also share the characteristics of requiring a separate lithographic film and print run but are not strictly spot colours.