Wednesday, 26 October 2011

WIDFP? - Colour Systems

Technical, essential knowledge for control of your work.

  • CMYK (cyan/magenta/yellow/key black - 4 colour process). Subtractive - This is used in the most common printed process called litho or offset litho
  • RGB (red/green/blue - screen based) Additive.
  • Greyscale (Black and white continuous tone and any shade of grey, such as black and white photograph)
  • Duotone (when a continuous tone image is printed in 2 or more spot colours) - this term is also generally used when describing tri and quadtones.
  • Spot Colour (one or more specially mixed colours as opposed as a result of a CMYK or RGB mix)
  • Mono (like greyscale but with a coloured ink, ie: one colour and percentage tints of that colour, plus the colour of the material it's printed on)

  • CMYK Definition: Stands for Cyan Magenta Yellow and Key Black. These are four basic colours used for printing colour images. Unlike RGB (red, green, blue,), which is used for creating images on your computer screen.
  • CMYK colours are 'subtractive'. This means the colours get darker as you blend them together. Since RGB colours are used for light, not pigments, the colours grow brighter as you blend them of increase their intensity.
  • Spot Colour: Print technicians around the world use the term spot colour to mean any colour generated by a non-standard offset ink; such as metallic, fluorescent, spot varnish, or custom hand-mixed inks. (as opposed to obtaining a colour via mix of CMYK).
  • Greyscale: One colour black and all the shades of grey through to white (black and white photography is grey scale)
  • Monochrome (mono): Monochromatic colours are all the colours of a single hue derived from one colour and extended using the shades, tones and tints of that colour.
  • Halftone: This is a mechanical process (as opposed to chemical) for converting tonal values into a series of dots that although solid dots, when printed give the impression of continuous tone.

Color Models
A color model is an orderly system for creating a whole range of colors from a small set of primary colors. There are two types of color models, those that are subtractive and those that are additive. Additive color models use light to display color while subtractive models use printing inks. Colors perceived in additive models are the result of transmitted light. Colors perceived in subtractive models are the result of reflected light. (link)

The Two Most Common Color Models
There are several established color models used in computer graphics, but the two most common are the RGB model (Red-Green-Blue) for computer display and the CMYK model (Cyan-Magenta-Yellow-blacK) for printing. (link)

Notice the centers of the two color charts. In the RGB model, the convergence of the three primary additive colors produces white. In the CMYK model, the convergence of the three primary subtractive colors produces black.

In the RGB model notice that the overlapping of additive colors (red, green and blue) results in subtractive colors (cyan, magenta and yellow). In the CMYK model notice that the overlapping of subtractive colors (cyan, magenta and yellow) results in additive colors (red, green and blue). (link)

To reproduce full-colour photographic images, typical printing presses use 4 colours of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of more colours. CMYK refers to the 4 ink colours used by the printing press.

C is cyan (a blue-green colour), M is magenta (a reddish pink colour), Y is yellow and K is a pure black ink, the key plate or keyline colour. (link)

How does CMYK work?

Because process colour ink pigments are imperfect, pure black cannot be achieved by overprinting CMY inks.

Consequently, black (K) ink is introduced in addition to, or in substitution for, CMY inks. The combined value of all CMYK inks for a particular area or object cannot exceed a specified amount, or ink may not transfer effectively and printed sheets may not dry properly.

This specified amount, referred to as Total Area Coverage;(TAC), typically is limited to 300% for offset lithography using coated paper. Compensation for TAC limitation is accomplished during the separation process, by which RGB colour is converted to CMYK.

Spot Colour
A spot colour is a specially mixed ink using in printing. Spot colour inks come in a rainbow of colours, including some speciality inks such as metallic and fluorescent. Unlike CMYK or process colour which creates colours by laying down 4 layers of just 4 specific inks, spot colours are pre-mixed and you use one ink for each colour in the publication.

There are different brands of spot colour inks. In the united states, the dominant spot colour printing system is Pantone. The pantone Matching System or PMS consists of over 1000 colours of ink. Other spot colour systems include TOYO, DIC and ANPA. (link)

A spot colour is basically a pre-defined colour that can be reproduced at any time. There are many different colour references but the industry-standard formulas are provided by Pantone.

Many of the standard colours have names (Fig. 1), e.g. Pantone Purple, Pantone Process Blue etc., but the majority are referenced purely by numbers (Fig. 2). Incidentally, the 'C' stands for Coated.

Spot colors are simply colored inks. The range of ink colors available and means for specifying these inks depends on the color system used by your printer. Printing Services uses the Pantone colour system. There are over a thousand colours available in the Pantone color system. (link)

Spot colour refers to a method of specifying and printing colours in which each colour is printed with its own ink. In contrast, process colour printing uses four inks (CMYK) to produce all other colours. Spot colour printing is effective when the printed matter contains only one to three different colours, but it becomes prohibitively expensive for more colours.

Most desktop publishing and graphics applications allow you to specify colours for text and other elements. There are a number of colour specification systems for specifying spot colours, but Pantone is the most widely used. (link)

What is Pantone?
A popular colour matching system used by the printing industry to print spot colours. Most applications that support colour printing allow you to specify colours by indicating the Pantone name or number. This assures that you get the right colour when the file is printed, even though the colour may not look right when displayed on your monitor.

PMS works well for spot colours but not for process colours, which are generally specified using the CMYK colour model. (link)

A Colour Matching System, or CMS, is a method used to ensure that colours remain as consistent as possible, regardless of the device/medium displaying the colour. Keeping colour from varying across mediums is very difficult because not only is colour subjective to some extent, but also because devices use a wide range of technologies to display colour.

Many printers keep an array of base Pantone inks in their shops, such as Warm Red, Rubine Red, Green, Yellow, Reflex Blue, and Violet. Most PMS colors have a "recipe" that the printer follows to create the desired colour. The base colours, along with black and white, are combined in certain proportions within the printer's shop to achieve other PMS colours.

If it is very important to match a certain PMS colour in your project, such as when a corporate logo colour is used, you may want to suggest to the that printer purchase that particular colour pre-mixed from the ink supplier. This will help ensure a close match. Another possible reason to buy pre-mixed PMS colours is if you have a very long print run, since it can be difficult to mix large amounts of ink and keep the colour consistent through several batches. (link)

Colour Separation
The act of decomposing a colour graphic or photo into single-colour layers. For example, to print fill-colour photos with an offset printing press, one must first separate the photo into the four basic ink colours: CMYK. Each single-colour layer is then printed separately, one on top of the other to give the impression of infinite colours.

This type of colour separation, mixing three or four colours to produce an infinite variety of colours, is called print process colour separation. Another type of colour separation, called spot colour separation, is used to separate colours that are not to be mixed. In this case, each spot colour is represented by its own ink, which is specially mixed. Spot colours are effective for highlighting text but they cannot be used to reproduce full-colour images.

Traditionally, process colour separation has been performed photographically with different coloured filters. However, many modern desktop publishing systems are now capable of producing colour separations for graphics stored electronically. This capability is essential if you want to create full colour separation if you want to create full-colour documents on your computer and then print them using an offset printer. You don't need to perform colour separation if you are printing directly to a colour printer because in this case the printer itself performs the colour separation internally. (link)

Multi-tone Images
There are 4 types of multi-tone images in all. Each one contains the number of inks as the name implies:

  • Monotone greyscale images printed with a single ink
  • Duotone greyscale images printed with two inks
  • Tritone greyscale images printed with three inks
  • Quadtone greyscale images printed with four inks (link)


Duotone is the generic name for multitone printing, which can be done with two, three or four inks. This process requires that the press be set up with special inks, usually PANTONE-designated colours, instead of the standard CMYK inks used for process color printing. Usually the images are printed with a dark base colour and a lighter second colour, overprinted to fill in, tint and tone the photo or graphic.

Duotone colour mode in Photoshop supports between one to four colours of ink. Because home inkjet printers are far better at printing full-colour images than black and white, many photographers and designers use Photoshop Duotone processing to improve their artwork/prints. Photoshop's Duotone mode is also commonly used for creating sepia toned prints, using black and yellow inks, sometimes with a third colour.

Rich Black / Plain Black

On a computer monitor, there is only one way to represent black. When there is no light coming from the monitor, the screen is black.

In print there are many different ways to represent black. The simplest is "plain black," or 100% black ink (0C, 0M, 0Y, 100K). However, you can also create a "rich black" by printing other inks along with black. There are many different possible ink combinations - the most common "rich black" contains percentages of all 4 inks: 63C, 52M, 51Y 100K. This particular variant owes it's popularity to Adobe Photoshop - when an RGB file is converted to CMYK, areas that are absolute RGB black (R0, G0, B0) will wind up with this combination, unless certain default settings have been changed. Other possible flavors of "rich black" are "Cool Black" (60C, 0M, 0Y, 100K) and "Warm Black" (0C, 60M, 30C, 100K).

Measuring the colour values
The solution to this problem is to measure the ink values in the CMYK (referencing Photoshop's info palette), and create a matching colour in the page layout program. Care must be taken that the rich black in the CMYK image is actually continuous; move the cursor around and make sure that the ink values don't fluctuate. Then create a new separated CMYK color in Quark using the Edit>Colors dialog box; you can call it "Rich Black." Assign that colour to the background of the picture box. Check your work by printing paper separations before sending out for film.

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