This post briefly introduces the two colour modes CMYK and RGB before moving on to the Pantone Matching System. It will then touch on mixing colours in the printmaking and letterpress workshops at LCC. It is meant as a small guide to help you decide when to use which colour mode and how to go about mixing the perfect shade in a print workshop.

RGB is an additive colour space. The overlap of its three primary colours, Red, Green and Blue, results in white. This is because the RGB model is based on three light rays which are projected onto a surface.

CMYK, on the other hand, is a subtractive colour space. The combination of its primary colours, Cyan, Magenta and Yellow, results in ‘black’ – or rather, a dark, muddy brown. The K in CMYK stands for Key, referring to the key colour or plate.

On Screen

A monitor or TV screen generates three colors of light: red, green and blue. The different colors we see are due to different combinations and intensities of these three primary colors. This means that RGB is used for anything displayed on a screen, so if you are preparing images and colours for websites, they should be in this mode. It has the added benefit of reducing your file size, because it contains one colour channel less. We will take a quick look at channels a little bit further down.

When you import images from your phone, camera or a scanner, they will by default be set to RGB. Most printers, including your standard home printers, can interpret RGB data. They will convert and output it as CMYK, but this will usually lead to duller colours. In order to have control over your printed output, always convert your images and files before printing – ideally while before designing! You can check and change your colour space in Adobe Photoshop under Image > Mode.

Hexadecimals, which you may have come across in web design, are basically shorthand for RGB. A hexadecimal is made up of a hashtag and six character. The first two characters stand for red, the third and fourth for green, and the last two for blue. Values range from 0-9, followed by A-F. F is the most light, while 0 represents the least: so #ffffff equals white, and #000000 equals black. 

In Print

Digital printing and offset lithographic printing predominantly use CMYK for standard four-colour printing. Modern printers like for example the HP Indigo additionally use orange, violet and white.

All visible colours are made up of cyan, magenta, yellow and black. The four colours are printed in detailed patterns of differently sized dots to create the illusion of all colours. When you look closely you can determine the dot sizes and angles at which they are printed.

In order to achieve a real, dark black, designers and printers usually specify a rich black. This is solid black overprinted with a combination of one or all other CMYK primary colours. This can also create hues of warm and cool black.

The distribution, size and angles of these dots matter. A standard combination of angles is Yellow at 0° or 90°, Black at 45°, Magenta at 75° and Cyan at 15°. When two colors are less than 30º apart there is a risk of moiré. There are lots of resources on the internet about different dot types and screen angles, for example on The Print Guide.

CMYK is not exclusive to digital and lithographic printing – in fact CMYK colour separations, or approximations where they are replaced with slightly or wildly different colours, are common in screen printing, risograph printing and even photo etching. 

The angles you may need for CMYK screen printing or photo etching can depend on additional factors such as your screen mesh, as the wrong angles could lead to Sawtoothing:

Below are two pieces of work by our own specialist screen print technician Lisa Chappell. Lisa works with four-colour printing across many processes.

RGB in Print?

There is research into recreating RGB imagery in printmaking, for example using Spectraval pearlescent pigments by Merck Group. These are suitable for screen, gravure and flexo printing. Red, blue and green are overprinted on black to create lighter shades where they overlap.

Pantone Matching System

The Pantone Matching System is a numbering system for colours in graphic design. This system is widely used to define solid colours, for what is also knows as spot printing. In this process, a single colour is formulated rather than mixed by overprinting CMYK. 

There are plenty of reasons why you might use Pantone colours:

• For more colour accuracy when printing, especially across different jobs, or when different printers being used
• To define specific brand colours that can always be accurately reproduced through spot colour printing
• Pantone allows for bright and bold, saturated colours; even neons, pastels and metallics
• You can colour match accurately across other Pantone Systems – for example for textiles

The Pantone Formula Guides, or fans, are an international go-to reference for colour specification and verification. Each colour swatch has a specific name and recipe, which tells you the base colours it is mixed from. This is what we use in the printmaking workshops to determine how to achieve that perfect shade.

If you work a lot in inDesign and prepare books for print, my main tip is to set up a Preflight profile that checks for allowed colour spaces and image resolution. That way, you can check the entire document in an instant. I have one set up for print, and one for digital publishing, where only RGB is allowed. You can choose to only allow spot colours for example, if you are defining only Pantones for a print job.

Mixing Pantone colours for printmaking

When mixing colours for printing, there are several things that will affect your mix:

• The type of inks. Oil based and water based inks will require different recipes
• The mixing medium. in screen printing for example, this will determine the opacity of your ink
• The printing method. How thick is your layer of ink going to be?
• The substrate. What you are printing onto? Is it absorbent, like uncoated paper or fabric? Is it glass? Is it light or dark?
• The lighting in the room will affect how you view colour. Remember ‘the dress’ that went viral?
• How new is your reference Pantone swatch? If it has been exposed to light and is old, its colour will not be accurate anymore.

All of these factors mean that the recipe can only get you so far. To accurately mix your colour, always mix a small amount first until you have your formula right. That way you won’t waste a lot of ink and you will be able to keep your mix simple more easily.

Keep testing as you go along. The easiest way to do this is to have a small sample of your substrate ready. Dip your little finger into the ink you have mixed and then dab it onto the substrate. Keep dabbing your finger, so the ink layer thins out, as this will represent your final outcome more closely. 

The letterpress workshop at LCC uses the same inks as offset lithographic printing. Our range is called Saphira by Heidelberg, and the workshop stocks all the base colours you need for mixing any Pantone shade.

Resources:
Pantone
The Print Guide