The Secret Science of CMYK 

What’s the big deal with CMYK? Why can’t you just open an image in Photoshop, select Mode and convert to CMYK? It’s that easy, isn’t it? I wish it was but the truth is creating a high quality CMYK separation requires a particular skill-set and how you decide to convert your image from RGB to CMYK or CMYK to CMYK can make or break the quality of the printing job. With so much misunderstanding about this topic it is difficult to determine what is the correct path to take and on top of that printers don't have the resources to teach you.

Standards have been developed to assist printers achieve more consistency on sheetfed presses. GRACoL is an implementation of the ISO 12647-2 printing standard, with metrics, methods and characterization data developed during seventeen research press runs at nine US printing sites over eighteen months. The goal of this initiative was to ensure greater consistency and predictability from one press run to another. Consider the implications of standardized printing: easier and more consistent CMYK conversions, realistic soft-proofing and more accurate desktop proofs. No more custom press profiles, thank god!

Below is a short list of the tips and thoughts on the subject of creating the best CMYK file:

1. More responsibility has moved down stream into the hands of the content creators. This means you, the designer, needs to learn about how to create a decent colour separation. 
2. CMYK is device dependent. Meaning there are many flavours of CMYK. One size does not fit all.
3. Repurposing a file from RGB to various CMYK colour spaces (flavours) makes the most sense. Keep your master file in the largest colour space possible, RGB and repurpose it as needed. 
4. Not honouring an embedded profile in Photoshop is really stupid and can cause significant degradation to a conversion!
5. Lightness and darkness are the biggest variables on press and is often referred to as Dot Gain or Tone Value Increase (TVI). Every ICC profile has embedded within the conversion TVI compensation curves for the target paper. GRACoL has less TVI than a SWOP 3 paper. But on screen they will look very similar. 
6. Learn about Total ink coverage (TAC) and how to check your images for over-inking. 
7. Paper should be considered the fifth colour in CMYK printing. Paper white/colour has a Huge impact on the outcome. Be aware of the power of the fifth.
8. Asking your printer how to create a good CMYK separation is like asking your IT staff for the servers’ password. It won’t happen.
9. Supplying a proof from your RGB file printed on a desktop inkjet is asking for trouble unless it has been properly calibrated and profiled or runs through a software RIP. Printing default RGB produces darker, richer blacks, more vibrant colours, not achievable on press.
10. Don’t supply proofs on Photo media that is glossier and whiter than the press sheet. If the paper you are suppling has the word Photo in the name chances are it is not ideal for proofing. i.e. Photo satin, Photo gloss as photo papers contain a lot of brighteners. 
11. Give the client the bad news early by showing them a realistic proof. Set sensible expectations.

Basic Target colours:

• Catch light, reflection or spectral highlights: an area with no detail C=0, M=0, Y=0, K=0
• Highlight: Lightest area with detail should be C=4 Y=2 M=2
• Mid-tone Gray: A neutral 50% gray should read C=50 M=42 Y=42 K=6
• Shadows: Darkest area with detail. (Dependent on your total ink limit) For SWOP the numbers are C=75 M=68 Y=67 K=90. 90-95% is the maximum amount. Anything more will likely fill in – no detail.
• CMYK is a small colour space and enhancing particular colours is required to ensure your images have the necessary punch or shelve appeal but going too far and you loose detail and shape. Learn what colours contaminate certain colours – colours opposite on the colour wheel are contaminates and removing that colour will increase the saturation.
• Cyan: Skies are made up of a lot of Cyan and a little Magenta. Keep Yellow low in these colours. Yellow will just dull the blue sky.
• Reds: Often needs a bit of or Black for detail. Y=100 and M=100 makes a Yellowish red M=100 & Y=90 is a better red.
• Blue: Too much Magenta will push the colour to purple. Keep the Magenta component under 80% for a rich blue.
• Green: Grass and leaves require at least 10% Magenta - remove too much magenta and your grass will have a day-glow look.
• Skin tones: You need a third cyan and equal amounts of magenta and yellow. Yellow can go a few points above magenta but the opposite (higher amounts of Magenta) yields a sunburned look.

Angus Pady is a colour management solutions provider and G7 Expert. He is located in Toronto, Canada and can be reached at 1-844-4-COLOUR or Angus@ColourManagement.ca