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Thursday 15 November 2012

Colour Theory - CMYK / RGB

Why would I ever design in RGB for printed material that will be printed in CMYK ― or even CMYKOG (Hexachrome®)?


The benefits of designing in CMYK = 

★ When designing in RGB, there will have to be a conversion to CMYK at a calculated time near the completion of an image. Sometimes, the colour of the image can change appearance due to this conversion. If you work directly in CMYK, there will be no such conversion ― and therefore, no colour loss.

Why doesn't my print out look like my screen?!
First, let's cover what the letters mean:
  • CMYK is a four color print process consisting of Cyan, Magenta, Yellow and Black
  • RGB is used for on-screen images consisting of Red, Green and Blue. It is the basic color model on computers and is used for Web graphics, but it is not used for print production.
  • PMS (Pantone Matching System) is a solid color matching system. It is a purchased color/ink, which is blended to provide a consistent and perfect match to the colors specified in the Pantone color guide. PMS inks are generally used in offset printing and limited other processes.
Because what you see on your monitor is not the same as the printed page, designers use PMS colors as a standardized point of reference.

CMY(K) colors work by "subtracting" or absorbing light reflected from a white piece of paper. With no ink or toner on the page, 100% of the light is reflected back to the eye and the paper appears white. By adding 100% density of each C, M, Y, (ink, toner) to a sheet, they absorb all the (white) light and we perceive the color as black (the absence of all color). By varying the amounts of Cyan, Magenta and Yellow, we can produce a immense number of colors, however, not as extensive as the gamut of color produced in the RGB method. While 100% CMY will produce black by themselves, black toner/ink is often added to them to provide more density and smoother transitions.
RGB works in the opposite way. Your computer monitor is essentially black, and light is added to produce color. When all 3 colors are added at 100%, it will produce white.
Both CMYK and RGB can approximate Pantone colors, some more satisfying than others. CMYK and RGB colors generally differ from each other. While some CMYK and RBG colors translate to print very well, others such as vibrant blues and oranges may print substantially different than what appears on screen.
Many color critical facilities spend thousands of dollars calibrating and setting up very structured and specific workflows to render the closest approximation from screen to print. Even in the most exacting environment, you are still viewing a CMYK color approximation on a RGB device.

What can I do when designing for print?
  • When designing for print you should use CMYK or PMS color palettes. When designing for the web or PowerPoint, you should use RGB color palettes.
  • Pantone produces formula guides and chip books that show PMS colors and how they appear when produced using CMYK inks. This is a helpful resource to use when selecting PMS colors for a print piece, which will enable you to see what the PMS colors will look like once printed on a CMYK printer.
  • Provide hardcopy print out or a piece that has been printed using the same artwork of how you expect the color to appear. This will give the printer a guide on how you expect your prints to look and they can adjust the printer to come as close as possible.
  • Ask for (and allow time in your timeline for) physical color proofs to be provided to you.
  • One easy and basically free method to aid in preventing costly errors down the production line is to always convert your workspaces to CMYK. By doing this it should reduce the instances of these issues occurring. Also keep in mind that NOT all monitors are not calibrated the same, therefore the colors will display differently on all monitors and monitor types (such as CRT vs. LCD). You will see more vivid colors on a standard CRT monitor and more muted and sometimes washed out colors on an LCD monitor.












The range of the Adobe RGB (1998) colour space, the sRGB (or small RGB) colour space and the common newspaper CMYK colour space. sRGB’s range is much smaller than Adobe RGB’s. Working in the Adobe RGB colour space would result in much brighter colours. The range of the CMYK colour space is much narrower. Especially for this newspaper, the white in CMYK mode isn’t white at all. It’s more of a dirty brown.

The cover of a brochure for a well-known Dutch beer brand. Adobe InDesign’s “Separations Preview” shows the RGB image in CMYK. Scene 2 shows the parts that will be highlighted using a glossy ultraviolet coating. Scene 3 is the part that will be embossed. Scene 4 shows all of the colors combined. (The combined image looks a bit weird because the UV coating and embossed parts have been given a extra spot color so that the printer can keep them separate from the full-color artwork).

When you export the document to PDF, the RGB images will convert to CMYK, and all of your spot colors will remain unchanged. I recommend that you check the color separations in Adobe Acrobat to make sure that everything that needs to overprint has been set to “Overprint” (Advanced → Print Production → Output Preview).


Colour Models
The purpose of a color model is to facilitate the specification of colors in some standard generally accepted way. In essence, a color model is a specification of a 3-D coordinate system and a subspace within that system where each color is represented by a single point.

But as you can imagine, mixing CMYK inks is very different than mixing RGB lights. For example, if more lights are added to RGB it produces brighter colors where as adding more ink in CMYK produces darker colors. In RGB, White is defined with the maximum value of each color channel (R:255,G:255,B:255), while in CMYK mode it’s defined as a complete lack of color (C:0%,M:0%,Y:0%,K:0% ).

RGB / Onscreen-Screen vs CMYK / Print
Example: A photo might end up on a computer, which will have a display connected to it. The computer's display profile might be "I have less red than I should". The profiles get compared and the extra red from the camera might cancel out the display's lack of red. An adjustment would need to be made to display the photo as well as possible. That's a massive oversimplification, but the basic theory stands.
A similar thing happens when you print. The idea is that a profile is attached to the image at the beginning of the process and it's kept to the end. This means conversion can be done as the image is displayed or printed as a non-destructive action, rather than processing the file needlessly several times, reducing quality.
Check your document is properly set up for CMYK print work...
Follow these guidelines when creating documents for print:
  • For Illustrator: on the New document window, make sure to select “Print” in the New Document Profile dropdown.
  • For InDesign: on the New document window, make sure to select “Print” in the Intent dropdown.
  • For CorelDRAW: on the New document window, select “Default CMYK”  in the Preset Destination dropdown.
  • For Photoshop: on New document window, select RGB as color mode and set resolution to 300 dots per inch. Immediately after that, turn on CMYK color imitation (View > Proof colors). When done, convert your final document to CMYK color mode (Image > Mode > CMYK > Flatten) then save the file under a different name. Use this file to finalize your print design in Illustrator, InDesign or CorelDRAW. Note that you can also create CMYK files right from the start (just pick CMYK mode on the New document window), but this will prevent you from using certain effects and functions.


A colour management system identifies the RGB and CMYK colour spaces that are crucial to your work— those belonging to your scanner, monitor, and printer. Descriptions of these devices are appropriately named profiles, or also referred to as characterisations


RGB vs CMYK workflow
CMYK is the printing standard and should generally be used.  An RGB workflow may prove beneficial if you use hexachrome (6 color) printing since hexachromes’ color gamut is wider then CMYK. That said, hexachrome printing is not the general printing standard due to additional printing costs so if your not going to use this printing method RGB images should be converted to CMYK.  If you don’t do it, this will occur at the printer and be out of your control (some colors may shift dramatically).


Deliver Correct CMYK printing files to client
The industry standard for CMYK file delivery is a properly prepared PDF:

  • For Illustrator: Go to File > Save As > “Adobe PDF” from the Save As Type dropdown box. A window will open prompting you to select PDF settings — all you need to do is select “Illustrator Default” from the Adobe PDF preset dropdown box, then hit Save. This will produce PDF file which is both ready for print and editable by Illustrator.
  • For InDesign: Go to File > Adobe PDF Presets > Press quality.  A window will appear allowing you to adjust many prepress settings, but you can safely hit Export and In Design will create a proper PDF file for professional printing.
  • For CorelDRAW: Go to File > Publish to PDF , then pick “Prepress” from the PDF preset dropdown box.  This will produce print ready PDF file which can be opened by any Adobe application as well.
In addition to this PDF file, you may export your design to other file formats requested by client (usually EPS or AI), but make sure to communicate that PDF is basically all they need because it can be opened and used by virtually any design software.

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Leeds College of Art. Graphic Design.
 

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